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?
Joe Morden in Babylon 5. I do not know much about the actor, but he was cast perfectly.
Or Bester. Koenig did a great job of making that character hateable.
Truthfully, while i thought he was a great villain, i think Bester was even better. Walter Koenig played him perfectly. But, the main thing that made him better was his complexity. He believed he was doing the right thing even when hunting down his fellow telepaths, and on some occasions, he was right, and rogue telepaths were a threat. So, sometimes Bester was in the wrong, and sometimes he was right, which in my opinion made him a better villain than one who is always evil.