The sequence was storyboarded by Bill Peet, who writes that he drew inspiration from having just had his own first child at the time. Awwww.
Dumbo finally discovering that his handicap-his large ears-give him the unique ability to fly. And the final scene where he loses his supposedly "magic feather" but still flies due to sheer will power, amazing everyone in the process and then punishing everybody who was ever mean to him: the circus ringmaster, the clowns, the other elephants...
The end where they wave goodbye with their trunks.
Dear god, the entire ending. Dumbo flies down into his mother's arms, covers her face with kisses, and throws his ears around her in a hug. Meanwhile, one of the crows is calling out, "Happy landings, son!"