Aren't Darth Vader and Hannibal ranked as the first and second best film villains of all time (respectively) by the American Film Institute? Whether or not that's true, they're both particularly bad ass.
Top 5 go:
1. Hannibal Lecter
2. Norman Bates
3. Darth Vader
4, Wicked Witch of the West
5. Nurse Ratched
That's pretty much my top 3, but I'd put in Alex from A Clockwork Orange and HAL from 2001: A Space Odessey in my 4 and 5 spots.