The Godfather I & II beyond all...

To chose between Goodfellas and Casino, I wouldn't know which one to pick. There are some similarities. I don't know.

I saw two movies about John Gotti, "Gotti" and "Witness to the mob" by Robert deNiro producing. They are good. Another one: "Hoodlum" about Dutch Schultz.

Carlito's Way and Scarface with Pacino.

And (if it is a mob movie): Analyze This, I'm really looking forward to Analyze That. (the first one was better than Mafia! with Christina Applegate)

Not a mob movie, but a crime movie: The Usual Suspects, my all time favorite.