If we're doing personal picks:
1. Emperor Palpatine (Ian McDiarmid - Star Wars: Episodes I, II, III, and VI
2. Bill the Butcher (Daniel Day-Lewis - Gangs of New York)
3. Anton Chigurgh (Javier Bardem - No Country for Old Men)
4. KHAAAAAAAN (Ricardo Motherfuckin' Montalban - Star Trek II: The Wrath of Khan!!!)
5. Darth Vader (James Earl Jones / David Prowse - Star Wars: Episodes IV, V, VI)
6. Norman Bates (Anthony Perkins - Psycho)
7. Michael Myers (Various - the Halloween series)
8. The Terminator (Arnold Schwarzenegger - The Terminator)
9. Biff Tannen (Thomas F. Wilson - Back to the Future / Back to the Future Part II)
10. Roy Batty (Rutger Hauer - Blade Runner)
11. HAL 9000 (Douglas Rain - 2001: A Space Odyssey)
12. Lo Pan (James Wong - Big Trouble in Little China)
13. Dr. Christian Szell (Laurence Olivier - Marathon Man)
14. Dr. Hannibal Lector (Anthony Hopkins - The Silence of the Lambs)
15. Patrick Bateman (Christian Bale - American Psycho)
16. Count Dracula (Bela Lugosi - Dracula ('31))
17. Amon Goeth (Ralph Fiennes - Schindler's List) - Hell, Nazis in general, or at least in Steven Spielberg films
18. Bill (David Carradine - Kill Bill)
19. Hans Gruber (Alan Rickman - Die Hard)
20. Nurse Ratched (Louise Fletcher - One Flew Over the Cuckoo's Nest)
Also, Cocaine in Boogie Nights.