I just rewatched season three...Faith and The Mayor are my favorite villians, however, I don't consider Faith to be a *true* villian, and I don't think she was "evil" or anything. But Buffy was sort of a bitch to her when she first showed up, the girl was already unstable...
But anyway, The Mayor made a great villian just because he acted so nice and it was funny.
Vamp Willow...um, I loved Willow pretending to be herself-as-a-vampire, but the actual vampire Willow was a little unimpressive, except for when she was breaking the guy's fingers.