I almost want to pick Warren now, he's rivaling The Master atm. Well, more him, Jonathan, and Andrew together as the Nerd Trio. Their argument over James Bond is one of my favorite moments of the season. Also when Warren's not killing people or trying to rape them the Trio actually lighten the season up, as when they're not around the season is dark and depressing
EDIT: I have my list now.
1.) Warren - Despite the scumbag he is he has some comical moments with Jonathan and Andrew, while how evil he is shows that he is not too far below his five predecessors as a villain (despite being human and the show seeming to want us to feel bad when Willow kills him)
2.) The Master - Sinister but kinda cool villain, played by Mark Metcalf
3.) Mayor Wilkins - For the reasons OwnerMan says
Fourth place is kind of a tie between Drusilla, Adam, and The First, and last goes to Glory, I don't know why. Incidentally the top three have my favorite death scenes, in order
(I know how it must look putting Warren as my favorite villain and a female villain as the least, but if I was a misogynist like Warren, I wouldn't be able to watch Sailor Moon, My Little Pony: Friendship is Magic, or Winx Club, let alone enjoy them, and however Buffy felt about it, Warren's death scene is one of my favorites of the series)