Here's my list...
Bonnie and Clyde.. Movie was like the first film to really depict graphic gun violence... It's dated but it's still worth the watch. The 2 lead performances are great as well as Gene Hackman in a supporting role as Clyde Burrows brother...
Menace II Society.. I feel like alot of people overlook alot of the subtle things in the movie because of all the gangster shyt that goes on in it... Cain IS a gangster but he has opportunities, even if they are small... but he's so boxed in and blinded by his environment that he doesn't see them.. that's what separates American street cats from 3rd world country street cats who are pretty much homeless and desperate...
City of God.. 3rd world gangsters, again unlike Menace.. these guys are killing for crumbs... and have little to no hope of getting out of their environment, they arn't blinded by what's around them.. they see it, but have almost no opportunities to get out.
Goodfellas
Godfather Pt II
Scarface
A Bronx Tale
Outrage 1 & 2.. They're newer flicks but they really give great insight into the japanese yakuza culture.. it's an honest look at the politics, greed, lust for power, the cycle of violence and how law enforcement plays a big part in it... I'm pretty sure there are earlier movies depicting the Yakuza, that I need to see.. that I havn't.
American Me.. great inside look at the origins of prison and The Mexican Mafia.
With a bat-man.. 
i wish they played old movies around my way more often. only thing i've had the chance to see was Raiders Of The Lost Ark and when they put Jurassic Park in 3D.
