In the sushi restaurant; the as-yet-unnamed Boo slips away from Sulley and goes to the other end of the room. Keep in mind she's a tiny, adorable toddler, she innocently says "Boo!". Absolute panicked bedlam ensues among the monster patrons...
Would not be nearly as awesome without Mike's reaction:
You know what, let's watch my favorite part again, sha-a-a-a-all we?
The International/Interdimensional Door Chase.
Heck, the very existence of the door vault is this. The DVD commentary states that the vault contains 35 million doors.
This sequence is titled "Ride of the Doors" in the soundtrack - which is also pretty awesome!
Mike slamming a door on Randall, with an entirely appropriate quip to boot:
Mike: I hope that hurt, lizard boy!!
At the very end, when Sulley swings on one of the doors and leaps across to the door Randall has escaped through.
Boo finally overcoming her fear of Randall and beating the crap out of him as he's about to push Sulley out the door.
Sulleytearing down a metal door to save Boo. Many parents watching that scene can relate to how he feels.
Following that scene above, the Scream Machine is about to be used on Boo and then Sulley bursts in with a roar and moves the machine away from Boo, before tossing it at Randall and Waternoose, pinning them against the wall.
How about Celia, who's spent the entire movie pissed off at Mike for abandoning her after the sushi restaurant is infiltrated, and who now has a vice grip on Mike as he's being carted away by Sulley, demanding he explains what's going on. When he does, she's skeptical at first until she sees Boo over Sulley's shoulder, which shocks her to the point of letting go, then sees Randall chasing after them like Mike had said. The crowning moment of awesome is when Celia creates a distraction for Sulley and Mike by announcing that Randall has beaten the all-time scaring high score, leaving everyone around Randall to mob him in congratulations so Sulley and Mike can get away.
Celia: Go get 'em, Googily-Bear.
Waternoose deserves mention for catching up to Sulley and Boo and managing to rip open a door that Sulley had secured shut with a metal pipe, all the while proving how terrifying he can be.
This brief moment deserved a mention
Waternoose: Because of you, I had to banish my top scarer.
Randall: Nah, with this machine, we don't need scarers. Besides, Sullivan got what he deserved.
Waternoose: Sullivan was twice the scarer you'll ever be!
Mike and Sulley's plan in general. As they're riding the door where Waternoose and the CDA are waiting to apprehend them, they must think of a plan. Mike throws a sock at the CDA and they chase him while Sulley runs with Boo while Waternoose chases him leading to the engineered confession that was taped by Mike with the CDA watching. And they thought of all that on the spot!
How about the fact Mike spent a lot of time, effort, and some cuts from the door shreds, just to make that happen for Sully. That's true friendship.
The third instance of the 2319. George got ANOTHER human sock on his fur and just as Charlie is about to call for the CDA again, George stuffs the sock into Charlie's mouth and throws Charlie into the kid's door and walks away happily while whistling. After getting his fur removed TWICE, it was completely reasonable, awesome, and hilarious.