I'm voting for Faith. While the Mayor was SO cool and funny, I really liked the way Joss thought up her character.
She was the dark slayer -- something Buffy probably wouldn't have been sure existed until she saw Faith kind of flirt with the idea of being evil. She brought out another side in Buffy that scared her, and for a while, Buffy wasn't as Ms. Responsibility as she normally was.
I liked the way they took the lore of BtVS, the essence of the show, and flipped it around to make her the bad slayer. Very cool.