Going by strict canon, that would be Anakin Skywalker or Darth Revan. The problem with Anakin was that he never used his full potential. He would have been capable of utterly insane feats if he'd bothered to focus on his mastery of the Force. However, he focused more on being a soldier and being in love. And, when he was locked away as the Vader we know and love, he was cut down to about eighty percent of his potential. (Of course, I'm pulling that off of my memory of Wookiepedia.)
It should be noted that 'bringing balance to the Force' was NOT making it so that there were two Sith and a few Jedi. The Dark Side itself was the imbalance, and by killing the Emperor, Anakin destroyed the last vestiges of the Dark Side in the galaxy, bringing the Force into balance at last. This is Lucas' version of the Prophecy of the Chosen One, and we should keep in mind that he's declared all post-ROTJ stories to be non-canonical.
People like to say Starkiller. While he was powerful, he had to struggle to win against Vader, and got himself killed by the Emperor. Also, remember that Exar Kun once destroyed an enemy starfleet by crushing them between two stars. Other old-time Sith have done things like destroyed planets or driven entire populations mad. The fact of the matter is that Starkiller only seems impressive because this is the first time we've seen his kind of power in motion.
