I voted Angelus, although it's a close call between him and the Mayor. What I liked about Angelus was that he just really was this badass evil vampire, and Angel's relationship with Buffy gave the whole storyline overall a nice extra touch, because of the difference between Angel and Angelus etc. What I liked about the Mayor was, again, the fact that, despite being weird and overly chirpy, he was really totally ruthless and evil. That's what I need in a Big Bad. Glory and Dark Willow both kind of lacked the evil ruthlessness, because their motives were, selfish, yes, but not wreaking havoc just because it's fun. The Master could have been interesting, but he was sadly underdeveloped. Same for the First. And don't even get me started on Adam.. 😛