In the X-men universe, there's a system of measuring the power levels of individual mutants, from Epsilon to Alpha, with the added Omega class being at the top as the creme de la creme of powerful individuals. There are eleven mutants that have been classified as being omega-level (according to Wikipedia, the number varies depending on where one looks); Jean Grey, Rachel Summers, Nate Grey, Vulcan, Hope Summers, Iceman , Mister M, Quentin Quire, Elixir, Legion and Franklin Richards. Some, such as Storm, have been listed as possible omega-level, but it's not clear as to what that actually means. In this post, I aim to give a brief summary of the powers and background of each of the eleven listed characters, taking note of possible similarities therein.
Rachel Summers is the daughter of Jean Grey from an alternate dimension (Earth 811, Days of Future Past) and another long-time host to the Phoenix Force. Therein, mutants ended up being considered a deadly threat to humanity, and at a young age her entire family and all her friends were killed virtually before her eyes in an attack on Xavier's, and Rachel was trained by the government as a 'hound', tracking down mutants to be killed. Though she was eventually able to overcome the immense trauma endured from her horrific past, it took Rachel a long time to come to terms with her suffering, even after managing to prevent the course of events that led to her abuse from occurring. During the course of her first appearance on Earth 616, Rachel ended up becoming a host to the intergalactic entity that is the Phoenix Force, largely through her grief at the death of her mother (and with the help of a Holoempathic crystal that was imprinted with Jean). I have, in the past, stated my belief that she is entirely too fixated on her mother, Jean Grey (I wrote at least one post on the subject), and her mother issues are undoubtedly an integral part of her character. Rachel is both a telepath and a telekinetic (just like her mother, another reason for her obsession, perhaps), and has been stated to possess strength therein roughly equal to her mother's, clearly marking her as an Omega level mutant. Ironically, Rachel also bears the weight of her mother's crimes as Dark Phoenix; an alien civilization known as the Shi'ar happened inhabit the solar system that was eaten by the cosmic entity's rage-fueled space-rampage, and (understandably) decided that the Phoenix Force needed to be eliminated (not all Shi'ar lived in the destroyed solar system, it's stated that their civilization spans many systems). As a result, the Shi'ar killed almost every member of the Grey family in a bid to ensure that the Phoenix not find another host. Rachel and her maternal grandmother (who denounced Rachel after the event) were the only survivors, and the Phoenix ended up returning despite the slaughter enacted by the Shi'ar death commandos. Up until recently, Rachel was in space, fighting alongside Havok to help stabilize the Shi'ar empire, but after the events of Age of X, she seems to be half-stuck on Utopia.
Next on the list is Nate Grey, son of Jean Grey and Cyclops from the Age of Apocalypse. Bred by Mr. Sinister in a bid to overthrow Apocalypse and clear the way for his own conquest, Nate is the ideal offspring of the Summers and Grey bloodlines, and he has been stated to possess powers that not only surpassed Jean Grey, but rivaled that of Dark Phoenix. In his final assault on Apocalypse, Nate was sent to Earth 616 by the M'kraan crystal, where he encountered the X-men, including Jean. He died after defeating Anti-man, but came back to life during Dark Reign and ended up falling victim to Norman Osborn and Dark Beast; Cyclops has sent the New Mutants to collect him, and they are currently attempting to free him from the grasp of Sugar man (As of New Mutants #24). As a telepath and telekinetic, Nate is unsurpassed by any character in the Marvel universe. It was once stated that he could easily shatter the moon or destroy all of Asia, and had the ability to deflect the most powerful of attacks, even ones with enough energy to destroy planets. He can also transform his entire being into psychic energy and possess other people. He can use psychometry, utilize the astral plane in a variety of ways, emit electromagnetic pulses at will, fly, create holograms with his telekinesis, use said telekinesis to give himself abilities like super-strength and super-speed, has precognition and clairvoyance, pass through solid matter, and manipulate objects as small as an electron or atom with ease. He was supposed to die at the age of 21, when his powers would consume his body, but in a battle with an alternate-dimension version of himself he managed to thwart this defect. Throughout his appearances in the world of comics, Nate has proven that telekinesis seems to be the single most useful ability if used properly.
The final character for this entry is Vulcan, who is the first individual on the list so far that doesn't go by his birth name and the penultimate of the Summmers-Grey family members listed. Born Gabriel Summers, he is the youngest child of Katherine and Christopher Summers. His brothers, Havok and Cyclops, are both powerful and prominent Alpha-level mutants with noteworthy abilities ( Adam-X is, apparently, the half-brother of Vulcan, Cyclops and Havok, but this- as well as Adam-X himself- are, in general, ignored by the comic book community these days). However, he didn't grow up with his family; Vulcan was torn from the womb of his dead mother by the Shi'ar that had captured his parents. Raised as a slave and aged to adulthood, he managed to escape enslavement and ended up joining a team put together by Professor X to rescue the X-men from Krakoa, the living island. The team was captured by the island, and only he and Darwin survived, although they remained in a state of suspended animation until M-day. Upon awakening, he sought to destroy the entire Shi'ar empire; however, due to a long sequence of events, he ended up becoming their emperor. A team of X-men was sent to thwart his attacks, and he eventually fell during War of Kings. Like his brothers, Vulcan absorbs and re-channels energy; however, he is on record as being able to absorb any form of energy, and can also manipulate virtually any type of energy. He is also the only Summers brother who is able to hurt his siblings with his abilities (Havok and Cyclops' powers do not hurt one another). Most of the time, Vulcan utilizes his abilities to create energy blasts and absorb attacks; however, he has also shown that he can somehow fly in the vacuum of space, repair neurological pathways with electrical impulses, and manipulate and even suppress psionic abilities. Though his manipulation powers are psionic in nature, he does not actually possess any innate telepathic or telekinetic skill or potential, he is merely able to utilize the energy already exerted by others. His absorption powers have their limits, however, and high-level energy projection drains him rapidly; he can still manipulate energy while drained, but is noticeably weaker. Of note is the fact that he is susceptible to physical blows, but is able to regenerate parts of his body at will to an uncertain extent (he couldn't regenerate his eye). Unfortunately, Vulcan is a megalomaniac, and his past has turned him into a relentless and selfish individual with a thirst for power.