Hands down, the open revolt of the school against Umbridge, Peeves "determinedly wrenching" off one of the chandeliers, and McGonagall passing by only to tell him "It unscrews the other way." The HP books are often at their most memorable when JKR just lets the humor out of the weirdest places.

And you gotta love both the Pensieve stuff and Ron becoming more confident.

Character-wise, Heroic!Neville carrying Hermione toward the end. I don't ship 'em, but that was a lovely grace note for Neville's character, who appears to be the Samwise Gamgee more and more of the HP universe.

- Madox