For me, it would be GLaDOS.
Portal 2 spoiler
She deleted the one remaining human part of herself because it was just too much to deal with. She releases you at the end of Portal 2 as a path of least resistance measure because “killing you is hard.”
What about you, what is your favorite media villain?
Abijah Fowler from Blue Eye Samurai, one of the only four white men in Japan who might be the father of the protagonist. He’s one of the few who manage to challenge the protagonist in combat, but you’re not just watching for that. The dialogue for the show is top notch, but any time he comes up you know you’re in for a treat. My favorite is still “Are you–still alive? Why?”, but here’s a reasonably spoiler-free scene.