Movies Only - Darth Tyranus aka Count Dooku. The best acting job in Ep 2, backed up by some delicous ambiguity as to what he's really after (read Stover's novelization of Ep 3 to see what I mean).
Expanded Universe Geekery - A vote for Kyp Durron, amoral destroyer of worlds and one of the only Jedi in existence with the potential to ever match Grandmaster Skywalker. A pity he's recently recieved the royal brush-off in favor of 'Darth Maniac' and The Sword.
Yes, I prefer my Jedi/Sith precariously balanced between inner peace and personal corruption like normal people are. Anyone who's 100% good or 100% evil gets boring after a while, and not just in Star Wars.