I will go for Depp and Crowe. They both can transform themselves into the characters they are playing so you don't even notice it's them. Depp is a master at nailing even subtleties of accents. When you see Deniro you can't get around the fact that he's Deniro. Pacino has played Pacino for the last 20 years. Also good: Duvall and Hoffman.
Samuel L Jackson or Christopher Walken.... how many bad movies do people watch just to see them speak?
Wow, great to see all the Norton love, I jumped on that bandwagon right after The People vs. Larry Flynt (just Primal Fear was a bit too small of a sample size, for me). More recently, I really think Eric Bana is great. I thought he was awesome in Black Hawk Down, and although Hulk was meh, he really picked it back up in Troy (anyone else pulling for Hector) and Munich. Those are definitely the first two I would look at were a casting a movie with no other information.
i'm gonna go with pacino for the last thirty years because his best is amazing and the first two godfathers are the greatest movies ever. i think russell crowe is far and away the best actor right now with honarable mention to depp and norton.