I find it hard to choose any one villain, so I'll pick a few.
The Auditors: What can you do when the laws of physics personified decide that you need to die?
Ipslore the Red: It's not just that he cheated Death, or decided to plunge the Discworld into a war that could have very well destroyed it. He mentally rapes his son.
The Archchancellor's Hat: More mental violation and megalomania.
Vorbis: Rewrites his own personal reality inside his head, believing his own BS.
Teatime: Obvious, isn't it?
The Cunning Man: No eyes, makes people hate witches, and basically makes people hate everything.
Yami: Wanna play a game?
Ushio: What sort of game-
Yami: First one to die loses!
Yami: Look! the Queen of Spades!
Ushio: What does that do-
Yami: You lose!
Ushio: *falls* UAHHHHHHHHH!
-YuGiOh Season Zero Abridged