The Departed springs to mind, Training Day has West Coast gangbanger ganstas in. Gangs of New York has gangs in, so I guess they would be classed as gangsters. All are quality movies.
My favourite films are mostly Gangster movies, Scarface, Godfather, Casino, Bozz N Da Hood, Mennace II Society, Carlitos Way. They were some of the first movies I got when DVD came out (Scarface was my first DVD and cost about £20), As soon as they are released on Blu Ray I will buy them again. I guess I am the reason they make up new formats every few years!
I am pretty sure The Sporanos is coming out on Blu Ray this year so that will be quality too.