The scene when Troy appears on Gabriella's balcony in 1.
The looks on Mr Bolton and Mrs Montez's faces when they see Troy and Gabriella singing on stage.
Gabriella coming back and doing a duet with Troy after having left in "Every Day."
Gabriella teaching Troy how to waltz "Can I Have This Dance" on the rooftop garden and then telling him yes she'll go to the prom with him in "every language".
Also in the rooftop garden: The scene in the first film where Troy and Gabriella open up and admit to each other how much pressure they're both under and how lonely they are because of it. Troy even admits that the garden is the place he uses to get away from all the expectations. The fact he shares it with Gabriella - and obviously feels he can be himself not 'the basketball guy' with her - is heartwarming all on it's own.
The Wildcats reactions when Gabriella returns at the end of 3: Even though she screwed up and left, they're all just happy she's back. Pretty heartwarming for a character who started with no real friends.
Troy's graduation speech in 3. Tear Jerker, too, because damn, Zefron did it well.
Here's one involving the cast: They all stayed friends since the films, and when Monique Coleman was on Dancing With The Stars, Ashley Tisdale, Corbin Bleu, and Lucas Grabeel all showed up, brandishing pom-poms in East High colors to cheer for her.