In reply to Choss:
Donnie Brasco
Mean Streets
Worth seeking out, but better if you have a reasonable understanding of Sicilian Mafia history, is the Italian film Salvatore Giuliano.
and of course you're completely wrong about the Godfather trilogy - 1 and 2 are masterpieces, 3 would be a good film if not for the fact that it suffers from comparison to the first two.