Spurred on the Dicaprio mention, I'll go on an Shakespeare roll: Dicaprio & Claire Danes playing two stupid kids in new Romeo & Juliet. Mel Gibson playing crazy in new Hamlet. Sir Ian McKellan playing a magnificent bastard in new Richard III.
John Slattery as Sterling Cooper in Mad Men. I just think the personality he brings to the character is unique and his delivery fits it perfectly. In the same vein, I would say Jon Hamm is THE Donald Draper. Hell, I'd say most, if not all of the cast is quite adequate.
I really liked Woody Harrelson as Tallahasse in Zombiland. I think he pulled off the bad assed bumpkin thing really well. But he also conveyed a lot of vulnerability.
His name is Roger Sterling, Sterling Cooper is the company. But I see what you mean. Daniel Day Lewis as Daniel Plainview.
Its already been mentioned, but it bears repeating; James Gandolfini as Tony Soprano. The little mannerisms, the way he moved, spoke, breathed. I think the character of Tony would of ended up quite different had anyone else been chosen for the part. Hugo Weaving as Agent Smith from the first Matrix. What stands out to me especially is the cadence of his voice and the way he enunciates in the movie. I'm sure someone else could of played Smith, but I doubt it would of been as memorable. Michael Clarke Duncan in the Green Mile. I think probably the finest acting of his career.
Also, V in V for Vendetta. Obviously anyone could have played that roll since you never see the actor, but I really think it was Weaving's voice and the way he speaks that just nailed it.
Not one mention of Joe Pesci in Goodfellas or Casino? What the fuck people: Spoiler Jesse Eisenberg as Mark Zuckerberg, in the Social Network: Spoiler He was robbed of that Oscar. Too bad the Academy is a sucker for underdog stories. Still a bit annoyed about that. Also - Andrew Garfield as Eduardo Saverin, the Social Network: Spoiler But my absolute favorite: Robert Downey Jr. as Tony Stark. Can anybody be a better fit?