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.
"What have you been doing since you stole that antique TARDIS of yours, since you first landed on Skaro? Shouting 'Look at me!!! I'm not fighting a war!', while you battle the Daleks all the way through space and time."