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.
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.
'Perhaps we can frighten away the ghost of so many years ago with a little ILLUMINATION! GENTLEMEN!'
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.
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.
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.