This guy killed Katar; an entire 'planet', with the Force. Enough said.
She is a contender, but she was a Jedi Master, and was the Order's Archivist before the first collapse, so she would have to be somewhat formidable.
We know that Dooku was a former Jedi Master, and was also an expert duelist and a master of the Makashi stance, which would make him deadly to other lightsaber-wielding combatants.
He was a mere apprentice, and yet was skilled enough with a Sith blade to kill a Jedi Master. I think that speaks for itself.
He was cunning enough to kill Revan once, and was able to muster an army that almost toppled the Republic. He was able to corrupt legions of Jedi Knights with the Star Forge, and was a powerful combatant.
This would be the obvious winner, except for the fact that he wasn't a Force user, which means that his decision to wield lightsabers as weapons was an extremely poor one. My vote would definitely have to go to this moron.
We know that Revan was one of the most powerful Force Sensitives who ever lived. He was also a strategic and charismatic genious. There's no way that Revan's in the running.
Mace was a Jedi Master, and was able to go toe-to-toe with Palpatine. He mastered the most difficult lightsaber stance, as well, so I don't think he's much of a weakling.
Are you kidding me? Sion was 'immortal'. He could not 'die', which means that he's one of the most powerful, as well as twisted, Force users who ever 'lived'.
He was the most powerful Force Sensitive of his time, was the son of Darth Vader, and resurrected the entire Jedi Order. Enough said.