Ron, just after kissing Hermione and with the Sword of Gryffindor in hand, yelling "LET'S GO KILL VOLDEMORT!"
A Very Potter Sequel
The introduction to "No Way."
Harry: For 11 years I was a little douche bag locked up under some stairs. But this year? I found out I'm a wizard, and I'm famous. I can fly and turn invisible, and I just traveled the fuck back in time! So fuck you Draco, how's that for a happy thought?
Lupin's introduction saving Harry from a Death Eater. "Take that, you bastard ass!"
All of the opening scene. "It's Not Over Yet" continues the grand tradition of Villain Songs being the best. In particular the moment when he pulls out the time turner; just listen to the pop from the audience as they realize what the sequel's story is going to be.
Hermione whips out the jellylegs jinx again, this time on Lucius as he's stealing Harry's Firebolt.
They twice take advantage of the fact that the third installment is a staged reading by having the narrator describe things too amazingly awesome to ever happen in a stage show. For bonus points, the narrator is famous voice actor Bob Joles.
"I'm Just a Sidekick" sounds awesome—Joey knocks the song out of the park.
A Very Potter Christmas
The Lang brothers obviously had a lot of fun at the expense of courtroom drama tropes, but that doesn't mean that they aren't able to get some awesomeness out of them, too. Sirius' inspirational speech to James over what being a Gryffindor means to him and James' dramatic final statements regarding why the aspiration to be brave is what makes Gryffindor the best house are actually pretty stirring whether you agree with their points or not.
Lily's last ditch effort to win the case by arguing that Peter Venkman is a Slytherin. It's a deft rhetorical move in- and out-of-universe, even with how ridiculous the nature of the argument may be.