Yeah I'm going for a list of serious movies - I'd forgotten about this post actually and now I look there are some really good replies. I added Carlito's Way because I thought that if I added The Untouchables, which is technically about a group of policemen, then Carlito's Way probably has as much right to be in the list. Anyway, here's a new draft of my list thanks to everyone else's comments:

The Godfather I, II & III
Goodfellas
Casino
A Bronx Tale
Key Largo
Carlito's Way
Donnie Brasco
Mean Streets
Untouchables
Bugsy
Gotti
Honor Thy Father
Lansky
Mobsters
Murder, Inc.
The Valachi Papers
The Brotherhood
Scarface 1932
Al Capone
St. Valentine's Day Massacre
Death Collecter/Family Enforcer
Mob War
The Youngest Godfather
The Last Don I & II
Hoffa

BTW Has anyone seen "The Sicilian" with Christopher Lambert? Should that be included? I seem to remember the book having Michael Corleone in it but I've never seen the film.