I'd probably say Visser Three or David, but I voted for David because he had less votes.
As a person, I hated David more than anyone in the series. As a villain, he was one of the most interesting, because he wasn't an evil alien slug hell bent on world domination, he was just a kid. A kid who was twisted by a series of cruel events in his life. He was a ruthless and cunning villain, but like most villains he was also arrogant, which led to his downfall. I guess the main reason I liked him as a villain was because it showed that you didn't have to be a Yeerk to be evil, humans are more than capable of doing terrible things.