I agree with all the Godfather votes (I & II), but I have to add some big love for my sentimental favorite, Miller's Crossing. <iframe width="560" height="315" src="//www.youtube.com/embed/G9ClOLR4dTo" frameborder="0" allowfullscreen></iframe>
certainly not what one thinks of as a classic "gangster" film, but American Gangster is one of my favorites. Denzel put on a fantastic performance
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The Original Godfathe with Marlon Brando. It's one of my all time favorite movies period. I loved GF 2 as well but the 1st was the best IMO. I thought Goodfellas was overrated and I like Casino more. I thought the Untouchables was damn good.
Godfather II followed very closely by Godfather I. Another film not mentioned yet.....Carlito's Way. Godfather II Godfather I Scarface Carlitos Way Goodfellas
The Departed is the American remake of Infernal Affairs, and was no where close to being as good. I can agree Infernal Affairs was a pretty good movie though. Triad film? I would probably have to say Young & Dangerous.
More underated Gangster type movies. The Professional (Leon) Year of the Dragon A Bronx Tale Road to Perdition Eastern Promises Chinatown A History of Violence The Town Bugsy New Jack City Blow State of Grace
I think it's because a lot of people have never seen it. It doesn't come on TV a couple times a year like Casino, Goodfellas, Scarface, and the Godfathers. I remember when I went and tried to buy the DVD in 2005 or so, (I didn't think of Amazon at the time), I went to multiple stores to find it. It just kind of came and went with minimal notice, not unlike the Big Lebowski did when it was originally came out. ON TOP OF THAT, I'd reckon that about half the people who watch it for the first time will need subtitles. Pretty heavy English nomenclature a lot of Americans wouldn't get first time around. Still though, DC's best performance. Skyfall is a close 2nd.
I'm going to say Godfather, Godfather II, and Once Upon a Time in America. You could pick either one of those. Miller's Crossing is right behind, and I love that film. I guess If I had to choose one right now to watch, it would be Once Upon a Time in America.