Chaplin
Better off silent
Another cliche, but I'm going with Heath Ledger's Joker, which was a mesmerizing bit of method acting.
Honorable Mentions:
Alan Rickman as Hans Gruber - Die Hard
Alan Rickman as the Sheriff of Nottingham - Robin Hood Prince of Thieves
Alan Rickman as Severus Snape - Harry Potter movies
Hugo Weaving as Agent Smith - The Matrix
James Earl Jones as the voice of Darth Vader - The Empire Strikes Back
Anthony Hopkins as Hannibal Lector - The Silence of the Lambs
Kevin Spacey as Keyser Soze - The Usual Suspects
Ricardo Montalban as Khan - Star Trek II
Kathy Bates as Annie Wilkes - Misery
There are a lot out there.
Honorable Mentions:
Alan Rickman as Hans Gruber - Die Hard
Alan Rickman as the Sheriff of Nottingham - Robin Hood Prince of Thieves
Alan Rickman as Severus Snape - Harry Potter movies
Hugo Weaving as Agent Smith - The Matrix
James Earl Jones as the voice of Darth Vader - The Empire Strikes Back
Anthony Hopkins as Hannibal Lector - The Silence of the Lambs
Kevin Spacey as Keyser Soze - The Usual Suspects
Ricardo Montalban as Khan - Star Trek II
Kathy Bates as Annie Wilkes - Misery
There are a lot out there.