Watch Means Streets, Taxi Driver, Raging Bull, The King of Comedy, The Deer Hunter, and even Heat for more DeNiro goodness.
There is one scene in particular near the end of Heat where there is only a front shot in a car of DeNiro's face, while he's contemplating doing one last thing before he gets on a plane to disappear.
Its completely dialog free, but its some of the best acting I have ever seen.