The scene in the sushi restaurant, and the TV interview with the "witnesses".
Horrified Monster: Well, the kid flew right over me and blasted a car with its laser vision.
There's also a witness, who is a large slug with a southern accent.
Slug Monster: I tried to run from it, but it picked me up with its mind power and shook me like a doll.
"It is my professional opinion that now is the time to PANIC!"
"Oh, hey. We're rehearsing a - a scene for the upcoming company play called uh, Put That Thing Back Where It Came From Or So Help Me. It's a musical." (for added hilarity, during the credits a musical with that name is enacted...)
Mike: Put that thing back where it came from or so help me, so help me, and cut!
In the bloopers:
Mike: Put that thing back where it came from or I'll poke myself in the eye!
Later, after Boo is missing:
Mike: What are you worried about?? She's out of our hair!! Randall: What are you two doing?! Random Employee: They're rehearsing a play! Mike: (singing) SHE'S OUT OF OUR HAIR!!
The scene in the locker room where Sulley just walks away, not even realizing that Boo is hanging onto his back until he goes around a corner and then runs away screaming. And just to make it even more funny, part of the Psycho theme is playing.
Sulley placing Boo in her room, he closes the door, turns around...and Boo's right in front of him! Becomes even funnier when he glances at her door in confusion and then at her.
A little context: Randall has just made Top Scarer of the day, only to be interrupted by a sudden scream from another door. Cut to Mike filling up several canisters, rapid-fire, from the aforementioned slumber party.
Randall's fate. ("'nother gator? Gimme that shovel!" *whack* *bong* *noises of pain from Randall*)
Sulley's Oh Crap moment when Boo is let loose in the restaurant.
Mike: Celia, please try to understand!! I have to...(Boo escapes from the bag)DO SOMETHING!!!
During the international/interdimensional door chase, Mike and Sulley end up in a tropical paradise.
Mike: Why couldn't we get banished here!?
Mike trying to apologize about not wanting to help rescue Boo, while an invisible Randall is beating Sulley up.
Sulley:(while trying to make Boo stop crying) 'Oh, he's a happy bear, and he's not crying, and neither should you, or we'll be in trouble, 'cause they're gonna find us. So please stop crying.
"It's 'Bring An Obscure Relative To Work' Day!"
The scene immediately after the restaurant chaos when Sulley and Mike are trying to deal with Boo in their apartment. For Sulley and Mike, it's like they've got a wild, lethal snake in the place. For Boo, it's basically the best game of peek-a-boo ever.
Sulley and Mike banter with each other throughout the credits song "If I Didn't Have You", and it's all very funny, but this bit takes the cake:
Sulley: Yes, I wouldn't have nothing if I didn't have you
Mike: I know what you mean, Sulley! Because-
Sulley: I wouldn't know where to go
Mike: Me too! Because I-
Sulley: Wouldn't know what to do
Mike: Why do you keep singing my part?
Also, when it sounds like they're dancing.
Mike: Don't you dare dip me! Don't you dare dip me! Ow, I should have stretched.
Sulley thinks Boo has been thrown into the garbage compactor and crushed into a cube and meets up with Mike;
Mike: Sulley! I think I got a way out of this mess, where's the kid? (Sulley, barely holding it together, holds up the cube) Sulley...that's a block of garbage. (Sees eye from Boo's costume) Uh, oh...
Sulley:(On the verge of a breakdown) I-I can still hear her little voice...
Boo, Off-screen: Mike Wazowski!
Mike: Hey, I can hear her, too. (Listens to cube)
Other Monster kids, off-screen: Mike Wazowski!
Mike: How many kids you got in there?
Mike going over different plans for getting rid of Boo.
"Hot-air balloon? Too expensive. Giant slingshot? Too conspicuous. Enormous wooden horse? Too Greek!"
When Mike reveals that he and the CDA were watching Waternoose's every move, he loops his "favorite part" of the security camera footage. It's Waternoose saying "I'll kidnap a thousand children before I let this company die!—I'll kidnap a thousand children before I let this company die!—I'll kidnap a thousand children before I let this company die!" For some reason, the repetition of that line is hilarious.
At the sushi restaurant when Mike tries to woo Celia right before Sulley shows up.
Mike: Why, just last week I was asked who I thought was the most beautiful monster in all of Monstroppolis and you know what I said?
Celia: What did you say?
Mike: I said, (sees Sulley out the window) Sulley?