Johnny Tran? He's about as evil as the Johnny from the Karate Kid movie.
Brick Top? Is he the one with the pigs? Good choice. How about the "If the milk turns out to be sour, I ain't the kind of pussy to drink it" guy?
Anyway, some of my favorites...
John Doe - SE7EN
Hannibal Lector - Hannibal
Darth Vader - Star Wars
The Alien - Alien Trilogy
Tommy DeVito - Goodfellas
Travis Bickle - Taxi Driver
Jack Torrance - The Shining
-Scott