it's not a movie, but JK Simmons in the great tv show "Oz" constantly gave one of the best performances I've ever seen out of any actor. when I see him on the show, I don't think "hey that's JK Simmons playing an aryan piece of poo", instead I believe that he is good ol' Vern, and always has being which is a great compliment to his incredible acting skills.
I think the only time when I noticed it was Simmons playing a character would probably be in the Spiderman films. never really bought him as JJ, but on "Oz", he was something else. the same guy that plays Emil Skoda on "Law and Order" plays the complete opposite on "Oz" and pulls off both so effortlessly. wonder how he does it.