Capone: I grew up in a tough neighborhood, and we used to say that you can get further with a kind word and a gun than you can with just a kind word.
Malone: You wanna get Capone? Here's how you get him. He pulls a knife, you pull a gun. He sends one of yours to the hospital, you send one of his to the morgue! That's the Chicago way, and that's how you get Capone!
Nitti: [About Malone] He died like a pig.
Ness: What's that you say?
Nitti: I said that your friend died squealing like a stuck Irish pig! Now you think about that when I beat the rap. [Walks off, combing his hair. Ness, enraged, grabs him and pushes him off the roof] Hey! What're you doing! Hey! HEY! [he falls, screaming]
Ness: DID HE SOUND ANYTHING LIKE THAT!?
All from The Untouchables 1987
Charlie: Look, kid, I - how much you weigh, son? When you weighed one hundred and sixty-eight pounds you were beautiful. You coulda been another Billy Conn, and that skunk we got you for a manager, he brought you along too fast.
Terry: It wasn`t him, Charley, it was you. Remember that night in the Garden you came down to my dressing room and you said, "Kid, this ain`t your night. We`re going for the price on Wilson." You remember that? "This ain`t your night"! My night! I coulda taken Wilson apart! So what happens? He gets the title shot outdoors on the ballpark and what do I get? A one-way ticket to Palooka-ville! You was my brother, Charley, you shoulda looked out for me a little bit. You shoulda taken care of me just a little bit so I wouldn`t have to take them dives for the short-end money.
Charlie: Oh I had some bets down for you. You saw some money.
Terry: You don`t understand. I coulda had class. I coulda been a contender. I coulda been somebody, instead of a bum, which is what I am, let`s face it. It was you, Charley.
On the Waterfront 1954
The above is great dialogue, but the performances in this scene are what elevate it to classic.