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Prof rescuing David and Megan when they're cornered by Enforcement. This is awesome on multiple levels; first, there's the straight-up badassery of dropping down through a dissolving ceiling, disintegrating bullets before they can hit him, carving a sword out of the wall, and taking down a fifteen-foot-tall armor unit with tensor-assisted punches. Second, Prof turns the crisis into an opportunity by identifying himself to Enforcement as Limelight and claiming the Reckoners they'd already tangled with as his "minions", thus salvaging a plan that had fallen apart. Third, in light of The Reveal of Prof's Epic status, you realize that he did all this at the risk of his own sanity and had to balance thinking clearly and fighting effectively with keeping the Epic madness under control.
David killing Steelheart.
David: ...you can only be hurt by someone who doesn't fear you. But such a person doesn't really exist, do they? You made sure of it. Even the Reckoners. Even Prof himself. Even me. We are all afraid of you. Fortunately, I know someone who isn't afraid of you, and never has been. Steelheart: You know nothing. David: I know everything.
The end of Mitosis, in which the citizens of Newcago come to David's aid at Tia's insistence, singing Mitosis' musicin order to kill him completely and save David's life. A user on a Brandon Sanderson fan forum presents a compelling explanation of the significance of this moment here.
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