After some deliberating, I think I'd have to go with the Mayor from Buffy. If nothing else, because of his relationship with Faith. On one hand, he's a psychopath who clearly used a broken young woman for his own benefit; on the other hand, he seems to actually really care about her. It's impeccably screwed up, and that's why I love the Mayor.  I prefer villains who seem unbalanced and complex, not... sarcastic and surly.

Second up is Spike before season five, maybe even in season five. We had all these cheesy vampires (and, in Angel's case, overly emo), then there was Spike. I don't even have any fangirly feelings towards him the way some people in fandom did - I liked Spike when he was still on the Dark Side, or at least... more Vampish than Scoobyish, to put it in Buffyspeak. He was just perfect. Then he fell in love with Buffy, and I got "bored now" very quickly.