Kelsi: Pianist here Ms. Darbus. Sharpay: (menacingly) You really don't want to do that. Kelsi: Oh yes, I really do. Sharpay: (Shocked exclamation and falls back in line)
Followed by Ms. Darbus'
Ms. D: Now THAT'S show biz.
Troy and Gabriella singing "Breaking Free" in front of the whole school. Especially when they get everybody dancing with them.
The very fact that both of them - a Shrinking Violet who is at a brand new school and a star basketballer under huge pressure with students and teachers watching his every move - had the guts to audition for a musical against experienced performers. Anyone who has been to high school can tell you that would be terrifying.
The ending scene after the credits when Zeke's cookies are so amazing that Sharpay falls in love with him.
This was a deleted scene, but apparently Ms. D and Coach Bolton were supposed to have a musical face-off in the Principal's office. That would have been a sight to see, especially since Alyson Reed (Ms. Darbus) formerly played Cassie in A Chorus Line. On Broadway. The woman has pipes, folks.
High School Musical 2
Ryan vs. Chad in a musical baseball game.
Ryan: You don't think dancing takes some game?
The fact that Ryan is actually really good at baseball, which impresses Chad.
Troy finds Ryan with Gabriella by the pool and Ryan makes a comment about the brownies Gabriella's mom makes. Troy shuts him down hard.
Ryan: Her mom makes the best brow— Troy: I know, I've had them.
Gabriella's confrontation with Sharpay.
Ryan standing up to his sister and making the East High Gang's participation possible deserves a mention. Not only is he awesome at what he does, he is also able to graduate from being Sharpay's lapdog.
High School Musical 3: Senior Year
Troy's song "Scream", which has scenes of him DANCING ON THE WALLS AND CEILING!!! That whole song in itself is a CMOA. It even doubles as Awesome Music.
Ms Darbus' conversation with Troy post-"Scream", wherein she proves she truly does understand the spirit of theatre and isn't just a poseur after all.
"The Boys Are Back", wherein they use metal bars as samurai swords and hang with what looks like guerrilla break-dancers.
Also, when mini-Troy and mini-Chad dance.
Speaking of upstaging: Rocketman is introduced as the annoying fanboy of fail. When he is told he actually has to go onstage as Troy in the musical, he goes into shock (his best friend suspects he stopped breathing). After freaking out some more in his dressing room and missing his cue entirely, he rocks onstage, completely steals the scene from Sharpay and actually turns out to be a good singer in his own right. This gets him an in-universe standing ovation. The best part is he completely ignored stage instructions and the costume department, threw on whatever he could find and had fun. He basically improvised, aside from the song. It is the single most badass and hilarious moment in the entire franchise. Try watching that part of the scene without immediately laughing and saying, "That was just awesome, good for him." Watch it for yourself.
The titular musical is choreographed in its entirety by Ryan. That's right, the guy who was Sharpay's shadow for one and a half movies is so good at what he does that he's put in charge of an entire musical and the results are amazing! He created a dance for Chad and the guys with basketballs, a number which is reminicent of Broadway. He even managed to make Jason look good on stage. The Juilliard people were so impressed they said "screw it" to the "one scholarship" thing and just gave him another one.
Ultimately, Sharpay has helped Ryan too. She's been hard on him repeatedly and, because of it, he's become brilliant at it...
Sharpay turning the tables on Tiara during the spring muscial performance of "A Night to Remember".
To elaborate on the former: previously, Sharpay had always been the bad girl trying to win the lead role, always backstabbing, manipulating, and buying her way to the top. Whenever Troy and Gabriella beat her out, she'd put a smile on her face and plot for next time. But in this movie, Tiara crushed her first. In one sweeping stroke, she replaced Sharpay as the villain- humiliating her, burning her acting, stealing her costume, revealing that she'd been playing Sharpay for the fool all along, and turning the light off on her dresser door. Sharpay completely breaks down. Then, for the first time in the whole series, she decides she's not going to sit there and whine when she doesn't get her way. She takes the challenge face to face, puts on her spare costume, and, in what could be considered the series' only fight scene, SHARPAY UPSTAGES HER!!! This is not only her CMOA, but her turning point from Villain to Hero - it's the moment Sharpay uses all the knowledge she's obtained being an Alpha Bitch for good.
Let's go through this upstaging scene by scene- Tiara is about to get to the heart of her song, when Sharpay's voice drowns her's out and down from the ceiling comes the woman in question, holding an umbrella. With a "Surprise!", Sharpay gets right into it, which leads to them jostling for attention. This includes shoving one of the background dancers out of the way - Sharpay is so experienced that she does it with one hand, while Tiara does it with two. The two of them keep fighting and get into a waltz before landing on a trap door. While Tiara is a little thrown off-balance and glances in worry at Sharpay, Sharpay just takes it in stride, probably because it's her brother in charge of the trap door. Then, when they're looking out of the trapdoor, they're still fighting for dominance with Sharpay on top.