The ultimate scare. To elaborate (seriously, please do not read this spoiler until you have seen the moment for yourself; I promise you, it is entirely worth the wait), after learning that his victory scare in the Scare Games was actually achieved by Sully cheating, Mike heads to the experimental door in the MU laboratory to scare the kid on the other side, proving his own merits. And he fails- and discovers that the door was actually to a whole summer camp full of kids. Panicking, he bolts, and the police are called. When Sully comes through to get him and they head back after a talk revealing that they're Not So Different, they discover that the dean has shut down the door on the other side for safety reasons. Mike quickly theorises that if they can just create a big enough scream from this side, they can power the door and escape. Only one problem: they're facing adults- a team of armed police officers no less- who are obviously incredibly hard to scare. And yet, between Sully's natural raw gift and Mike's genius skill at crafting the perfect tailor-made scare worthy of a horror movie, they manage it. And that's not all. Oh no. The pure power of their scare is so great, the screams they elicit fills up every single last scream canister on the other side until they burst from the sheer pressure and thereby blow a gaping hole in a highly secured laboratory, explode the door into charred splinters of wood thanks to it being overloaded, get through said door just in the nick of time- and do what is thought to be impossible: actually shock (maybe even scare) the unflappable Dean Hardscrabble of Monsters University, holder of the all-time scare record, who was even collected when her only souvenir from a lifetime of work was destroyed. That, dear viewers, IS HOW IT'S DONE.
A bit before the above moment, when Mike tells Sulley to "Stop being a Sullivan and start being you!".
Oozma Kappa surprising everybody by making it past the second challenge. What's not so awesome is when Roar Omega Roar invites them to a party ... only to humiliate them by making them look "cute".
Oozma Kappa's cross of misdirection, bluff, and sheer Refuge in Audacity to get past the library challenge. Remember, the only rule was not to get caught. Every other tactic was perfectly legal.
Mike: What are you doing?!
Terry: They said "don't let her catch you."
Terri: But they didn't say how!
What's even more awesome is that they apparently came up with the whole thing on the fly.
It also reveals that as much as they initially hinged on Mike's coaching, they are capable of holding off on their own. Notably after this scene, Mike starts to show more respect to Oozma Kappa's own abilities.
Little Mike sneaking past the scarer from MU and surprising him.
Squishy's scare technique during the final Scare Game challenge, which involves simply sneaking right to the child's bed and just staring at them with a hilariously creepy gaze.
Randall getting his scare attempt sabotaged, if you don't feel sympathy for him or his choices.
Oozma Kappa proving that they are confident scarers on the final event of the Scare Games. Even if Wazowski wasn't meant to be scary, Oozma Kappa got re-enrolled in the Elite Scaring School.
Sullivan and Mike working their way up through Monsters Inc.
"Frightening" Frank McCay (the monster who inspires Mike to become a scarer in the beginning) had a pretty darn impressive scare routine, involving clever tricks like scraping his nails on the kid's bedpost to misdirect them.
Much later, Mike uses the same tactic when he sneaks into the human world to prove himself. Even if his actual "scare" was lacking, he performed everything else flawlessly
"When you lose... no one will even remember you." "When you lose... no one will ever let you forget it."
Don during the final scare event when he's paired against Reggie Jacobs and Don ends up with the highest score.
Although we later found out that Sully tampered with the stimulator, indicating that Mike would not have the winning score, Mike's scare performance at the game counts. Mike has a fleeting moment of painful memories of monsters telling him he does not belong. He quickly cast aside the pain and immediately belts down his scare performance. To the point where the audience is truly fooled into believing in the OK's fair victory.
It wasn't just that. You could almost feel like Mike gathering up all the pain, humiliation, frustration and anger he's accumulated all these years and finally let it all out in one roar. Overall the roar sounded pretty intimidating, even if it might not have been on par with Sully's.
The visuals certainly helped here too; the way that shot was framed and lighted, combined with the roar, makes that one singular shot the only time in the movie that Mike actually comes across as scary. It's an impressive achievement both for Mike and the moviemakers.
Hardscrabble remarking that she will look for more "surprises" in her Program, indicating that Mike's and Sully's actions have changed the University standards for the better.
On the note of Hardscrabble, there's her Tranquil Fury-laced, utter verbal beatdown towards Mike and Sully during the Scare Exams.
The Monsters Inc Scarers epic Power Walk. This isn't just a Call Forward to the Monsters Inc. but it's also shows the young Mike a glorious vision of greatness.
Mike deciding to settle for "being ok." The film cleverly fools you into believing this is the overall message of the film, until Sulley rushes over and rebuffs it, declaring that Mike may have that unfortunate shortcoming, but by god, he is fearless and that's a greatness Mike shouldn't take for granted