This is a topic that has been circulating for quite some time, and I have had this question asked to me many times as well. I am tired of repeatedly answering this question, in part due to the fact that my opinion also rapidly shifts. In the future, when I am asked this question, I will link the inquirer to here, where I will also be updating this blog as my feelings on the characters change/material is changed. This is, of course, a Legends listing. I will create a Jedi list some time in the future.
It should be noted that the question of "Who are the top 10 Sith Lords" is ambiguous and comes in many forms, so I will address it in all these forms, and I will give an explanation for each choice.
Note: Obviously, this is just my opinion, and is not necessarily correct (although Palpatine remains canonically the most powerful Sith Lord by a variety of sources). Also, theoretically, the Banite Sith that we do not know of will also be in such a list, but I do not want a list filled with nameless and unknown characters, so I have included only characters with enough feats or accolades to actually warrant a position in the top ten.
This is a list referring to Force power only.
1. Darth Sidious - No explanation really required - sources have already identified him as the most powerful Sith in history and his feats reflect this. Nobody else is on his tier in this regard. Check his respect thread for his power feats and accolades if you are disbelieving of me.
2. Darth Plagueis - Listed as the most powerful Sith Lord up to and of his time by Darth Plagueis's back cover (not sure if it is canon, though), and this should be correct based on his showings which include atomizing others whilst he was heavily wounded and holding back.
3. Darth Caedus/Vitiate - Caedus is often thought of as a masterful duelist and fast combatant, of which he holds a record as two, but in terms of power, I would not place him that far up. Vitiate the most powerful Dark Sider up to and of his time (excluding the Ones and Abeloth, like all these lists are) by canonical sources, and is exceptionally powerful by virtue of his myriad of feats, notwithstanding his vast overrating which can be found almost universally.
4. Darth Nihilus - Nihilus is exceptionally powerful by virtue of his feats, such as casually stunning Visas Marr, Canderous and the Exile, given how the Exile was likely of considerable power by this time. His Drain feats are his most notable ones, but he also has good Concealment and Sense feats. There are two respect threads for him.
5. Darth Traya/Darth Nox - Traya is a very powerful yet underrated Sith Lords whose power feats should naturally carry the connotation of being one of the most powerful Sith Lords and Force users in general. Her TK feats are incredibly impressive, her Heal, TP, Cloak and similarly-related feats even more so, in addition to her extensive knowledge and versatility in the Force. Nox is another overwhelmingly powerful Sith Lord, whose feats include utterly stomping one of the single most powerful Sith Lords in the entire Sith Empire with telekinesis (comparable to the likes of Count Dooku as well), and effortlessly deflecting Lightning from said Sith Lord (Darth Thanaton, who has killed beings with his Lightning before), in addition to his myriad of other showings and accolades.
6. Darth Tenebrous - Very few showings, but what few showings he has demonstrated place him somewhat above Vader, though not by much. That said, he possesses a strong level of telekinesis and Lightning, immense levels of Barrier and Speed, as well as a powerful degree of Force Sense and overall knowledge and mastery in the Force. There is a respect thread for him I have created if you wish to seek out his feats.
7. Darth Vader/Darth Bane - Vader should come as no surprise, given the enormity of the scale of his telekinetic feats, which he is well known for within the EU community, and he possesses a number of showings in other power-related areas to afford him this ranking. Bane is extremely powerful in many Force-based depictions, which should elevate him above the likes of Tyranus, even though his telekinetic feats have some edges of doubt about them.
8. Darth Tyranus/Darth Krayt - Probably a tier behind Vader, if not on Vader's tier, but not quite his equal. Tyranus is exceptionally powerful in a number of regards and canon sources has identified him as one of the most powerful Force users in history. A respect thread has been made for him if you wish to see his feats. Krayt is also relatively knowledgeable and powerful. His telekinetic capabilities allow him to shatter pillars, he knows Dark Transfer and Drain, he has withstood Darth Wyyrlok's Memory Walk, deflected Wyyrlok's Lightning (utilizing Deflection against Lightning being a near-impossible feat) and so on, which should merit his ranking within the top ten.
9. TCW Darth Maul/Darth Malgus/Darth Zannah - Maul should only be a tier behind Dooku. His telekinetic feats significantly transcend that of his TPM incarnation, allowing him to utterly dominate Jedi on the caliber of Obi-Wan Kenobi. A respect thread has been made for him if you wish to see his feats. Malgus is a sheer brute powerhouse whose raw power is outstanding and has enough merit to be on the list. Additionally, Sidious has claimed that Malgus's battlefield feats, accomplished by virtue of power, have never been duplicated up to and of Palpatine's time. I have created a respect thread for him if you wish to see his feats. Although Zannah's prime has never been shown, her displayed feats up to DoE (including those of Sorcery, and her childhood showings) do reflect incredible power, which should merit her ranking at roughly this level.
10. Darth Malak/Darth Revan/Lumiya - Malak is a very underrated character whose power feats merit a level which significantly transcends that of beings of Obi-Wan's class, so he should be comparable to Maul and Tyranus. There is a respect thread for him on this site. For Darth Revan, prior to being redeemed, it's hard to identify exactly where he fits since he has very few feats at this time, and all we know is that he possesses extensive knowledge and is one of the most powerful Sith Lords in history, but I recall a source which elevates him beyond Malak, but of course, that iteration of Malak was very unlikely to be as powerful as he was by the time of KotOR, so he should be placed here, especially in part due to a lack of feats. In the case of Lumiya, she is fairly powerful across all areas, but more notably, is powerful in Illusion-based powers. Her mastery of the Force Phantom technique was such that she could create and sustain hundreds of them across the galaxy at once, whilst empowered by the Dark Side nexus of the Home.
There are many characters like Chratis, the Emperor's Wrath, Wyyrlok and so on who are close but not quite able to make it to the top ten. Characters like Marka Ragnos and Tulak Hord are mostly lip-service and a lack of living appearances makes it difficult to judge whether they live up to their hype or not. I am not including amped characters like Orbalisk Bane, but for the record, Orbalisk Bane would be on par with Plagueis or Vitiate. Likewise, Star Forge Malak is not included, but for note, he should be around Malgus's level.
This is a list regarding skill only.
1. Darth Sidious - Again, little need for explanation. His feats are simply better than the others'.
2. Darth Caedus - Contending with Luke in a realistic combat situation is a very good showing for Caedus. Barring some senseless inconsistencies, Caedus is certainly one of the most skilful combatants in being comparable to Luke, which renders him comparable to Palpatine as well.
3. Darth Tyranus - I was actually surprised myself when I couldn't bring myself to find another duelist more skilful than Tyranus, and I did not expect him to be here. Nonetheless, this is not a baseless assumption. Tyranus's dueling feats and accolades (which can be viewed in his respect thread) are just as astonishingly impressive and he should be ahead of others that I list here.
4. Darth Vader - One of the single most skilful swordsmen in the history of the galaxy itself, and having beaten a myriad of Jedi to support his placement here.
5. TPM Darth Maul/TCW Darth Maul - Just as skilful as each other, these incarnations of Maul are one of the single most skilled of Sith in the entirety of the Sith Order's history, and their feats include fighting competitively with some of the best duelists in the history of the Order.
6. Darth Plagueis/Darth Tenebrous - It is difficult to judge where Plagueis is due to a lack of feats on his part, but from what we have seen and what can be implied, he is still a very skilful combatant. I would not place him on par with Maul just because of his lack of likeness of lightsaber combat as a whole and a level of ambiguity, although he could be of roughly tantamount ranking in that regard - it is simply too hard to quantify his dueling skill. For Tenebrous - essentially the same reasons for Plagueis.
7. Exar Kun - Kun has beaten a number of Jedi such as Craydo, Sylvar, and Vodo-Siosk Baas, all of whom are featless, but the latter of which has been listed as one of the most formidable Jedi swordsmen in all of history. He has also stalemated Ulic Qel-Droma (and this was before Kun brandished his dual blades), a character whom could stalemate Sylvar even after the former was cut off from the Force.
8. Darth Malgus/Emperor's Wrath/Lumiya - Malgus has beaten a number of skilful Jedi opponents, such as Ven Zallow and Kao Cen Darach. The Wrath has beaten a vast range of characters, a number of which were beaten under circumstances which would make the feat more impressive, since most of the characters he or she has beaten are featless. Nonetheless, the showings should have merit, given how a number of them were accomplished whilst he was relatively inexperienced. Lumiya has mastered the lightwhip, a much more difficult to master but also more esoteric counterpart to the lightsaber. She was able to defeat RotJ Luke (an equal for Darth Vader) with the advantage of Luke having never faced a lightwhip before. After Luke became more experienced against such weapons and dual-wielded against them, Lumiya was defeated, but she was able to contend with Luke on other occasions, including as a Grand Master (although this is an inconsistent showing, but it is a decent one nonetheless).
10. Darth Venamis/Darth Revan - Venamis's capacity to contend with Plagueis should elevate him to a top ten standing, but he has no other appearances or showings so it would be extremely difficult to otherwise quantify his combat skill. Is he very skilled? Yes, but exactly where he stands I cannot say. I'll just place him here for now. For Darth Revan, he has beaten Darth Malak in a realistic lightsaber duel at some point, hence Malak's loss of his jaw, as well as having beaten Mandalore the Ultimate, whom, whilst a non-Force sensitive, has one-shotted Alek (Malak as a Jedi).
Tulak Hord is likely somewhere amongst the characters, but a lack of appearances is painful. The characters who spoke highly of him have a hyperbole record so whether he was actually as skilful as he was claimed to be is up to one's determination. Nox also has some decent skill feats, but I wouldn't put him or her as a top ten charcter just yet. Kas'im is fairly skilful, but probably lacks the feats to be top ten.
A reference to Dark Side knowledge and Force knowledge in general.
1. Darth Sidious - Again, no explanation really required. He has all the knowledge of the Banite Sith, plus a vast range of other sources. He is believed to know every single application of the Force, as per the DE sourcebook.
2. Vitiate/Darth Traya/Darth Plagueis - All wield tremendous reserves of knowledge, and Kreia has the edge of light-sided knowledge from her former Jedi heritage, during which she served as a historian who gathered and studied relics. Vitiate has an incredibly extensive life span, which affords him the capacity to delve into the deeper secrets of the dark side, and Plagueis has the benefit of all the accumulated knowledge of the Banite Sith.
3. Darth Revan - Revan has access vast range of lore such that his holocron appeared to possess much more knowledge than the entirety of the Sith Academy Library on Korriban, and his information sources stretch from the Trayus Academy to varying Sith manuscripts, holocrons, and other sources.
4. Darth Tenebrous/Darth Caedus - Tenebrous's status as among the last of the Order of the Sith Lords presents him with the opportunity of an amassed wealth of accumulated dark side knowledge. Caedus is knowledgeable as a whole given the diversity of his Force powers.
5. Darth Tyranus/Darth Zannah - Tyranus has studied many artifacts and has displayed a wide range of powers. Zannah has similar knowledge to Bane, but would have only gained more during her tenure as a Sith master, not to mention that her Sorceress capacities allowed him to uncover more information from holocrons than Bane did.
6. Darth Krayt/Darth Bane - Krayt and Bane both wield vast reserves of knowledge based off their large libraries of collected dark side knowledge and displayed powers.
7. Darth Vader/Darth Malak/Darth Nox - Vader possesses a wealth of Force knowledge and versatility. Malak knows a wide range of powers and has dabbled in arts such as Sith Sorcery, whilst Nox is simply very versatile and knowledgeable from what we have seen.
8. Darth Wyyrlok/Atris/Sorzus Syn /Lumiya- Likely not as knowledgeable as the above, but Wyyrlok is still knowledgeable in his applications of dark side knowledge. Likewise, Atris was once a Jedi librarian, and has studied from a wealth of dark side holocrons. We also know from the Knights of the Old Republic Campaign Guide that Atris wields many powers. As detailed in Book of Sith: Secrets of the Dark Side, Sorzus Syn is simply a very well-learned character in Sith sorcery, alchemy, and other dark side-related phenomena. Lumiya also had access to a large force of holocrons.
9. Exar Kun/Freedon Nadd/Darth Thanaton - Exar Kun wields a wide range of powers from holocrons he has studied, including Sith Sorcery, whilst Nadd has learned everything Sadow knew, as well as gaining knowledge from varying other Sith such as Adas. Thanaton has access to an entire library of dark side knowledge and lore, and is very knowledgeable in varying Sith/Dark Side powers and traditions such as rituals.
10. Naga Sadow - Sadow is considerably knowledgeable in the arts of Sith Sorcery and Alchemy.
This is a list which is not necessarily a realistic fighting scenario, just an amalgamation of all characters' full power sets and abilities. Therefore, a lack of skill or a lack of power can be harmful:
1. Darth Sidious - No real explanation needed.
2. Darth Caedus - His mix of power, knowledge, endurance, speed and skill makes him an extremely skilled, powerful and versatile fighter.
3. Darth Plagueis - A very powerful/knowledgeable Force user with incredible levels of speed and agility as well as strength to supplement his power, also being very skilled.
4. Darth Tenebrous - Skilled, but also extremely fast, agile, knowledgeable and powerful. Additionally, he is tremendously intelligent and is capable of calculating the future via mathematics.
5. Darth Vader - An incredibly durable, powerful and skilful combatant with a healthy mix of knowledge and speed added to complement his greatest aspects.
6. Darth Tyranus/Darth Nox - The Count is portrayed as a consummate duelist, is very fast, and is very powerful/knowledgeable. Nox also wields vast reserves of power and knowledge, and is a capable combatant himself.
7. TCW Darth Maul - A masterful lightsaber combatant and martial artist with high levels of all physical attributes as well as power.
8. Darth Traya/Darth Nihilus - Extremely powerful, but lack combative and other physical showings.
9. Vitiate/Darth Bane - The Emperor is incredibly powerful, but is not a physical combatant and lacks any lightsaber or physical capacity at all. Bane is not quite as powerful, but is still skilful and possesses many physical feats.
10. Darth Malgus/TPM Maul/Darth Krayt/Lumiya - A strong blend of power, endurance/pain tolerance and strength with reasonably decent skill makes Malgus a juggernaut who is not to be trifled with. TPM Maul isn't very high with respects to power, but is very formidable in terms of martial and dueling skill, and in all physical areas. Krayt lacks physical strength feats, but has fairly decent speed and durability showings, half-decent skill showings, and impressive levels of Force power, mastery and knowledge. Lumiya possesses a strong level of skill and speed at least approachable, if not equal, to the late Rebellion era-early NR era Luke. She also has a strong level of power and knowledge, especially in respects to Illusions such as her Force Phantoms.
Honorable mentions to the Emperor's Wrath, Malak and Zannah, who aren't too far behind the list.
1. Darth Traya - She is so interesting because of the way she challenges SW morality and philosophy, which makes her a very unique character. Her wisdom and knowledge is also particularly striking, although most probably don't like her due to the interminably long monologues she launches into.
2. Darth Sidious - Essentially the main villain in SW history itself, and the story of his manipulations and rise to power makes him perhaps the most epic of all Sith in that way.
3. Darth Tenebrous - Very, very underrated as a character. I find Tenebrous interesting because of his different capabilities and goals unlike many other Sith.
4. Darth Vader - You'd probably have to have him somewhere in your top 5 Sith list if you are an SW fan - no real explanation. The first SW character to radiate epicness.
5. Darth Plagueis/Darth Tyranus - Plagueis is just intriguing due to his character and the way it was explored by Luceno - to reveal his character and make a good story out of it, but maintain a shroud of mysteriousness. Tyranus is up here because Christopher Lee is underrated and an excellent actor.... and he has a good backstory, good characterization in sources such as the RotS novel, etc.
6. Darth Caedus - I always liked Jacen Solo, and the same applies for Caedus. It's hard to actually explain why, though.
7. Darth Revan - I'd think that anybody who played KotOR would understand why he is here. In short: he comes from one of the best SW video games. You can play the game to find out for yourself.
8. Darth Zannah - She unfortunately doesn't have the characterization she could have had, thanks to Karpyshyn, but she has much promise and potential as a character.
9. Darth Bane - He had good characterization in the first novel of his namesake trilogy, and it developed him into a strong character. Unfortunately, that couldn't be sustained in the next two novels due to Karpyshyn's preference of fanwanking over characterization.
10. Darth Malgus - Was developed very well as a character in Deceived, and his appearances the trailers alone personify his epicness.
My most hated/least favorite, number 1 is most hated, and gradually dwindling away.
1. Vitiate - An utter ripoff of Palpatine, and the development of him as a character is essentially reliant on his absence of character. Just.... no.
2. Savage Opress - He sucks. Too much of a brute, too poor a character. He's essentially "I'm going to punch you in the -" *gets his leg tripped on cloth*
3. Marka Ragnos - I literally dislike him because of the fanwank he receives. Just... uggh....
4. All Lost Tribe of the Sith - I just never got myself to like any of them. Vestara is probably the only one I don't dislike.
5. Atris (as Darth Traya) - The entire premise of her character in the game was to be a nuisance, really. Her stuck-up arrogance and hatred for the protagonist really made her the unlikeable character she deserved to be.
6. Darth Thanaton - He had somewhat better character than Atris, and more exposure to the audience through comics, but he was still essentially an Atris in the game.
7. Darth Bandon - Just wasn't developed enough as a character, and he felt somewhat unnecessary as a character.
8. Ludo Kressh - I liked Naga Sadow, so it would only be natural that in his rivalry against Kressh, that I would support Sadow against Kressh.
9. Darth Sion - He was a decent character in the way he was developed, but I just dislike his premise of being a complete and utter Force zombie. That, and he looks like he sleeps with vibroblades at night.
10. Qordis - Not necessarily a bad character, I just never liked him. To be fair, he was something of an a$$hole in the novel anyways.