Hmm, this is a tough one.
It's a tie between Min-sik Choi and Jack Nicholson.
Reasons? Min-sik Choi did an amazing job in the Korean film OldBoy (For those who aren't bothered by sex or violence, see this and the other Vengeance films. They are friggen brilliant.), and the scenes where he is still trapped inside the hotel room (Especially when he does that grin) he eerily reminds me of me.
Jack Nicholson because if you give him curly hair, he'd look about 90% accurate, and I think he's a good actor.