Hey just remembered one of the best cult classic gangster movies which doesn't get a lot of recognition and is seriously underrated - The Warriors, it's a little far fetched at points but is a great film to watch.
Definitely Once Upon a Time in America (long version) Having Deniro smile at the end after smoking opium was classic.
The best I've seen is Godfather and Godfellas. I love these documentaries too - Al Capone Scarface, The Gambinos and Inside The Mafia: The Godfathers