The Mean Streets is a great film.
Lacked convincing violence, but brilliant all the same.

The use of music too is brilliant and this was one of the first films to really use the device. It hasn't aged well but there are some brilliant scenes. Who's a mouk?

The Untouchables is a must see
boondocks saints
don't forget Once Upon a Time in America

Donnie Brasko is awesome. Johnny Depp is really, really great. It won me over as a Depp fan. Plus you got Al Pacino and Ray Liotta in it.

It's basically a movie about Depp's character, who is a cop and goes deep undercover to infiltrate a mafia type crime organization (Pacino is a criminal). He ends up basically forgetting he is undercover and becomes part of that lifestyle himself. Awesome movie.