Revan--I mean, what was it, centuries of mental assault by the Emperor?
Palpatine--Literally being able to coerce, manipulate, and outright deceive an entire galactic senate, the Jedi Council, etc.
Yoda
Luke
Exar Kun--even in spirit he was ridiculously powerful
Bane
Kaan
I'll add more later most likely. I'm not sure that I include Jacen. For whatever prowess he showed earlier in the EU and in spurts later on, he was taken in by Lumiya and the dark side like it was nothing, and continued to be the weakest, most ambivalent Sith in the mythos. The guy straight up did not know who he was or who he wanted to be and Jaina taking his head was a mercy.
Please stop bringing up the "centuries" thing. It's impressive that Revan withstood the Emperor and the Dread Masters, but the fact that he did so for centuries was solely because he had an unlimited reserve of Force energy to draw upon (Surik's Ghost). The novel outright states that because of Surik's Ghost, Revan was able to "do more than keep the Emperor at bay".
Pretty big red herring. Nowhere did I insinuate that it was Revan's only willpower showing or that his willpower isn't impressive. But the "centuries" argument is flat-out incorrect. That's all I was pointing out.
Well, that only implies he could hold the Emperor at bay without her deary. It's not as if Jedi ghosts actually have unlimited reserves of Force energy.
Anyway, Plagueis and Palpatine willed the Force out of balance, they win.
Corroborated with Palpatine's ability to spread his will across the galaxy, render the twenty-billion population of Byss "almost mindless" and effortlessly overpower the wills of Luke and Vader and I'd say Sheev is #1 certainly.
#1 Darth Sidious - with Plagueis willed the Force out of balance; spread his will across the entire galaxy; rendered the mindless a 20-billion population with his "oppression"; effortlessly overpowered the wills of Luke and Vader; able to fool Master Yoda with powerful illusions.
#2 Yoda - described as the most implacable foe the dark side had ever known; resisted the attempts of a host of Sith spirits to overcome him, told the ghost of Bane to go **** himself and resisted a joint attempt to dominate his will by Dooku and Sidious, all on a planet saturated in the dark side. Confronted his own dark side made manifest and overcame it; saw through the various illusions of the Force priestesses; was able to partially penetrate the fog of the dark side.
#3 Darth Plagueis - was literally able to will people to death by forcing their midichlorians to do his bidding; was also able to overcome Force resistant species, heal injuries, reverse aging and create life in this manner; and was at the forefront of the ritual that unbalanced the Force; mused that no Sith powers were beyond his reach that he couldn't achieve through sheer strength of will.
#4 Valkorion/Vitiate - broke the will of a Sith lord as a child; overcame the wills of 8,000 Sith lords in a ritual; with a fraction of his powers was able to break the will of a corrupted Revan and Malak; was able to break the wills of even the "the strongest of Jedi" such as the Hero of Tython & Tol Braga; rendered Scourge a crying pussy with a glimpse of his mind; gradually dominated the wills of everyone on Ziost; controlled the wills of his "Children" from light years away.
#5 Revan - was able to resist the attempts of Emperor Vitiate and the Dread Masters to dominate his mind over the course of centuries while waging a mental war with Viitate; through sheer will was able resurrect himself from the dead and keep his rent spirit together; was able to resist the corruptive powers of the Star Forge, Malachor V, Korriban; resisted the crushing effects of Nathema; as a rent spirit was able to maintain his presence on Yavin 4, a planet immeasurably strong in the dark side; telepathically dominated the minds of many of his followers.
I also feel Luke should be on this list but I'm not familiar with his feats.