Crowning Moment of Funny: Pretty much anything Slevin says, but if one had to pick a moment, it's when Elvis and Slow meet Slevin:
Slow: Tell it to the one legged man so he can bump it off down the road.
Or shortly thereafter:
Elvis: Nick, Slevin, Clark Kent, whatever the fuck your name is—the Virgin Mary herself could come waltzing up in here with her fine ass titties hanging out and everything and if she said your name was Jesus Christ, I still gotta take you to see The Boss. Know why?
Elvis: Orders. Now, you do know what orders is, right? Orders is orders.
Slevin: So I guess no one ever taught you not to use the word you're defining in the definition.
Elvis: (punches Slevin in the gut) Say something else! I will break your motherfuckin' nose, I ain't playin' with you!
The back and forth between Slevin and the cops when they grab him off the street for a quick word is incredible. Like many of Slevin's conversations, it ends with him getting punched.
Slevin vs. the mugger. It's just plain priceless. Especially when you consider that it didn't really happen! Slevin's even a smartass in his own mind.
Slevin: ...am I being mugged?
Mugger: (gives him an insolent glare and punches him in the nose)
The delivery of this line is nothing short of amazing.
"I'm a world class assassin, fuckhead."
The Boss is pretty damn scary and intimidating, but he does have a knack for being a Deadpan Snarker.
Slevin: I thought he was the Boss.
The Boss: (glances at his bodyguard) Why? Do we look alike?
A Funny Background Event: The entire time the Boss is explaining Slevin's troubling predicament, Slevin is trying to pour himself a glass of water. It seems to exasperate the hell out of the Boss if you watch his facial expressions, when he realizes he has been monologuing and Slevin's been trying to get a drink of water. Slevin even gets caught and still goes for it while the Boss begins to walk out of the room. Harsher in Hindsight, when you realizes it's because he himself is a hitman, plotting to kill the Boss. He doesn't care if he gets caught, because he knows there won't be any consequences for the Boss thinking he's a schmuck, since he's essential in Goodkat's "plan."
Slevin accidentally flashing Lindsay is funny enough, but the scene is immediately topped when Lindsay rushes back into the apartment a second time to try and catch another look. The smug look on Slevin's face is priceless.
Lindsay: I wanted to try and catch the next show.
Slevin: I don't go on again until eight.
The conversation after she sees him naked and returns for the cup of sugar also bears mentioning because it is also unstoppably adorable.
Lindsay: It's like something out of a Norman Rockwell painting.
Slevin: What? My penis?
Lindsay: The sugar. The penis thing's not very Mayberry.