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?
Might be more of a monster than a villain, but I’ve loved the Disney version of the headless horseman since I was a kid. That laugh still gives me chills