Casino...nobody mentioned this movie yet

Some oldies no one mentioned
Scarface - the original 1932 movie which Oliver Stone based his 'Scarface' on

The Public Enemy - Starring James Cagney

Little Caesar - Starring Edward G. Robinson

The Roaring Twenties - Starring Humphrey Bogart and James Cagney

Murder Inc.
St. Valentine's Day Massacre
Manhattan Melodrama (the last film John Dillinger saw)

And coming out soon "Public Enemies" starring Johnny Depp and Christian Bale amongst others

There are a ton of old