Puddleglum the Marsh-Wiggle's Shut Up, Hannibal!. He stands up to an enchantment which is causing him and the rest of the protagonists to forget not only their mission, but that the world beyond Underland ever existed - first by stomping on a fire with his bare foot, and second:
"Suppose we have only dreamed, or made up, all those things — trees and grass and sun and moon and stars and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up things seem a good deal more important than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well, it strikes me as a pretty poor one. And that's a funny thing, when you come to think of it. We're just babies making up a game, if you're right. But four babies playing a game can make a play-world which licks your real world hollow. That's why I'm going to stand by the play-world. I'm on Aslan's side even if there isn't any Aslan to lead it. I'm going to live as like a Narnian as I can even if there isn't any Narnia."
"...what remained smelled very largely of burnt Marsh-wiggle, which is not at all an enchanting smell." Awesome.
It's been pointed out that Puddleglum manages to do a Shut Up, Hannibal! while agreeing with every point the witch makes. He knows that what she's saying makes sense...and he doesn't care.
Puddleglum in general; like when he interrupts the Earthman's "Many have (blah blah blah)..." "Yes, we know. And few return to the sunlit lands. You are a chap of one idea, aren't you?" Or when Eustace and Jill tell him it's not his fault they missed the second and third signs, because he tried to stop them, and he says, "I'd no call to be trying. I ought to have done it."
Made even better with Tom Baker's delivery in the BBC adaptation.
The story of how Prince Rilian got lost in the first place, where Drinian goes to Caspian and confesses that it's his fault no one stopped Rilian from going back to see the green lady — and Caspian grabs an axe and goes to kill him, and Drinian just stands there ready to take it — and Caspian drops the axe and says that if he had to lost his son, he doesn't want to lose his best friend as well, and they hug. Crowning Moment of Heartwarming too.
Right at the end of the book, Eustace and Jill return to their school in full Narnian attire, swords and all, and prove that they have taken a level in badass in the course of their journey by terrifying the bullies of the school. Of course, the fact that Aslan comes too probably helps.
The unnamed city that seems to be the heart of Underland is awesome in a "Marco Polo" sort of way. The later discovery of Bism is a more spectacular one.