I voted Public Enemy because James Cagney is the shit.
I love all of his gangster movies including Roaring 20's, Angels with Dirty Faces, and White Heat.
Also, there's definitely not enough Chow Yun Fat mentioned here. Hard Boiled, The Killer, and A Better Tomorrow are just three of his best.
Finally one of the greatest gangster films that hasn't been mentioned is Little Caesar starring Edward G. Robinson. His portrayal of Rico is unparralled and set the standard for gangster movies throughout the 30's.
It's a fantastic rise and fall story.
