Naegi, Kirigiri and Togami in their Big Damn Heroes arrival in the final act. Some of the biggest damn heroes in the whole series.
Hinata's Awakening. Even better, the Mastermind wasn't expecting this in the slightest.
Hinata and Nanami destroying AI Junko's argument.
Monomi standing up to Monobear. It doesn't work in the end, but that doesn't undermine the sheer heroism of the act by one bit and proves the character's sincere devotion and willingness to stand up and fight for her "students" no matter what it takes.