This question has been answered by canon numerous times:
Darth Sidious, in all. Master of the seven lightsaber forms, is a master of them ambidextrously, moves at near-relativistic speeds, creates wormholes that rip through time-space continuum and destroy fleets/planets/moons, drains an entire planet with a population of 20 billion, mind-wiped an entire world's worth of people, and is immune to Nihilus' drain, has arguably the most powerful Force Lightning, has studied from numerous holocrons including Naga Sadow's, Bane's (which holds all of Revan's knowledge), Freedon Nadd's, even some Jedi ones, and much more. He is also a master of Essence Transfer and is the only one who involuntarily uses it.
He has been stated to be the most powerful Sith Lord ever numerous times:
Yoda went after Palpatine in the empty Senate chamber, but could not defeat the most powerful Sith Lord in history.
--Taken from The New Essential Chronology
Darth Sidious proved to be the grim culmination of a thousand years of Sith philosophy and teachings.
--Taken from Jedi vs Sith: The Essential Guide to the Force
When the Sith finally emerged from a thousand years of watching and waiting, they numbered—in accordance with the tradition set down by Darth Bane—only two. The most powerful of these was Darth Sidious, an ice-cold, diabolically calculating genius equipped with the strength of the dark side of the Force, as well as an enormous wealth of Sith artifacts, equipment, and knowledge.
--Taken from The Dark Side Sourcebook
And then there was Palpatine, of course: he was beyond power. He showed nothing of what might be within. Though seen with the eyes of the dark side itself, Palpatine was an event horizon. Beneath his entirely ordinary surface was absolute, perfect nothingness. Darkness beyond darkness. A black hole of the Force.
--Taken from Revenge of the Sith
Plagueis made a soothing gesture and explained in great detail what had taken place. Concluding, he said, "He threatened, too, to place you out of reach."
All the while Plagueis spoke, Palpatine was storming through circles on the narrow path, shaking his head in anger and balling his fists. "He can't do this!" he snarled. He hasn't the right! I won't allow it!"
Palpatine's fury buffeted Plagueis. Blossoms growing along the sides of the pathway folded in on themselves, and their pollinators began to buzz in agitation. FourDee reacted, as well, wobbling on its feet, as if in the grip of a powerful electromagnet. Had this human truly been born of flesh-and-blood parents? Plagueis asked himself. When, in fact, he seemed sprung from nature itself. Was the Force so strong in him that it had concealed itself?
--Taken from Darth Plagueis
Palpatine has spent decades studying the most arcane and esoteric Jedi disciplines. It is believed that he has mastered nearly all the known powers, previously unknown powers, and devises new ones at his pleasure.
--Taken from the Dark Empire Sourcebook
And many other times also.
Revan is nowhere near as powerful as Sidious, after reading the Revan novel (which gives him canonical feats) I don't see him being in the top 5, or even 10, for that matter, of either Jedi or Sith in terms of power.
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