Sulley opens the rebuilt door. "Boo?" "Kitty!". His expression says it all.
Not only that, it was Mike, who started off as a selfish Jerk Ass, going through all the trouble (and splinters) to rebuild the door in the first place. Now that'sfriendship. What makes it even sweeter is his apology to Sulley about the time necessary.
Mike: Sorry it took so long, pal. It was a lot of wood to go through.
The ending was such a moment of heartwarming, especially Sulley's final smile, that Pixar opted instead to do a prequel as opposed to a sequel, feeling they couldn't bring themselves to continue the story after that perfect moment.
Any of Sulley and Boo's scenes together, like when he helps her fall asleep by showing her that there's nothing scary in the closet.
Mike's evolving attitude. Through most of the film, he just wants to get Boo back out of his and Sulley's lives. By the end, he's willing to decoy the CDA to help save her from the bad guys. Whether you believe he did it for her or Sulley (or maybe both) that's a big change.
Boo's tiny little nod when Sulley reassures her when they're hiding under a desk.
When Sulley declares that "none of it matters anymore", Mike is visibly crushed. You can see Sulley regrets the words almost immediately, but he doesn't take them back. Mike then names several things he feels should matter to Sulley. "What about everything we ever worked for? What about Celia? And what about me?" That last one is when Sulley turns around and apologises - because Mike does matter to him.
The conclusion of the film: Seeing the monsters be reassigned from In-UniverseNightmare Fuel to entertaining kids and evoking laughter from them. Because the emotional energy they get doesn't have to be negative. The monsters get the electrical power they need, and the children they visit are made happy; everybody wins!
And it looks so much more fun to work in, now.
Made even better after the prequel. Oozma Kappa had to work hard to overcome their limitations during the "scream power" days. Now they can be the superstars.