My personal favorite line was in "Donnie Brasco," which was during the fugazi diamond ring situation. That was when Pacino said "How the fuck do you know that's a fugazi? You only looked at it for two fucking seconds." Below are some other funny ones. Please fill free to put more.

Richard Castellano
"Leave the gun, grab the cannolis" - The Godfather

Armond Assante
"You know what you both should do? You both should become CPA's. Yeah certified public assholes, this way you can have your blue shield and workers comp. On top of that you go pay your taxes" - Gotti

Unknown actor
"If she goes down on the old br$$$ knowing that the guy in the truck was watchin' then she's a whore and she can't be trusted" - A Bronx Tale

Joe Pesci
Funny how? You mean I make you laugh, I'm here to fuckin' amuse you? Tell me what the fuck is so funny about me" - Goodfellas