Mesrine was pretty badass.
Here are a couple more favorites courtesy of Denzel.
Alonzo Harris - "You're in the office, baby."; "This shit's chess, it ain't checkers"
Frank Lucas
Hmmm, no real gangsters yet?! As far as real life gangsters go, I've always been interested in the history of Carlo Gambino myself.
I'm also somewhat surprised no one mentioned "Lucky" Luciano or that guy from the Chicago Syndicate.