'Perhaps we can frighten away the ghost of so many years ago with a little ILLUMINATION! GENTLEMEN!'
That high note Christine hits at the end of the title song.
Her cadenza at the end of "Think of Me" is also quite spectacular.
During the production of "Il Muto", where the Phantom ruins Carlotta's performance by making her croak like a frog onstage, and then drives her offstage with his creepy laugh.
Carlotta herself gets one at the end of "Prima Donna" in the 25th anniversary. After being singled out as a target for the Phantom's ire, with the threat of "a disaster beyond your imagination!!!" still ringing in everyone's ears, she gives this◊ incredible expression before leading the entire assembly in one last, defiant, "ONCE MORE!!!"
Hell, during "Poor Fool, He Makes Me Laugh," even when the Phantom interrupts to threaten everyone (her in particular), and sabotages her performance, she keeps right on going. Despite being visibly terrified, she's determined to give her performance, Opera Ghost be damned. Say what you want about Carlotta, but that takes serious guts.
Um... Something about a chandelier at the end of Act I.
The Phantom crashes the masquerade ball at the beginning of Act II (in a spectacularly cool Red Death costume) just minutes after the other characters were expressing relief at being done with him for good. "Have you missed me, good Messieurs?" indeed.
"Why so silent, good Messieurs? Did you think that I had left you for good?"
The musical transition contributes greatly to the scene. One moment, the guests of the ball are happily singing the climax of "Masquerade", the next, the opening notes of the Phantom's theme thunder through the hall, silencing everybody.
Raoul really Needs More Love—especially for the graveyard scene, where he faces down an angry out-for-blood Phantom and calls him out on the whole "trying to force someone to love you via threats and kidnapping" thing. Oh yeah, did we mention the Phantom is hurling fireballs at him through all this? Who says the Vicomte de Chagny isn't badass? Not to mention swimming across the underground lake in order to rescue Christine, as well as begging her not to sacrifice herself for him. In other words, he was willing to die to ensure her safety.
Come to think of it, during the ENTIRE FREAKING MUSICAL, Raoul is the only character who never has any of the Phantom's shit. Even when he's threatened to be killed by him.
Subverted in the novel, where he's basically a spoiled young noble who distrusts Christine at the height of her dilemma, faints as soon as he sees Erik in the graveyard, and hopeless flops his rescue mission even with the Persian to help him.
After the Phantom presents Christine with his Scarpia Ultimatum — "His life is now the prize that you must earn. So, do you end your days with me, or do you send him to his grave?" — Raoul throws this back in the Phantom's face with "Why make her lie to you to save me?"
Even more badass when you consider that Raoul is almost certainly provoking the Phantom in hopes that Erik will kill him and, by the terms of the bargain, free Christine.
Christine doesn't just sit by and weep either; she snarls back at the Phantom "The tears I might have shed for your dark fate/Grow cold, and turn to tears of hate!" showing him clearly that she's not going to be a pushover for him any longer.
The unmasking scene. Lon Chaney's self-applied makeup was so incredibly well done, that some audience members in 1925 were reported to have fainted upon seeing his face. Still scary today, in a Jump Scare sort of way.
The Phantom and Christine's duet. Her showing off the beautiful voice she's garnered thanks to his lessons, him showing off HIS singing chops, aand them affirming their love for each other via the "Anges Pur, Anges Radieux" duet from Faust.