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.
I've lived for over 2000 years, and not all of them were good ones. I've made many mistakes. And it's about time I did something about that.
-The Twelfth Doctor, Doctor Who: Deep Breath