I would have voted for Vader if I had only seen "A New Hope" and "The Empire Strikes Back". He was completely evil there. On "Return on the Jedi" (spoiler) he supposedly went back to the good side, so he wasn't completely evil then. And then after watching the prequels, seeing as how he should have been named Darth Backstreet instead...
But this list needs Kim Jong Il from "Team America: World Police" and, of course, real life.