One of the best things in Order of Phœnix was the prank war against Umbridge and her Inquisitorial Bridage right after Fred and Georges Weasley's awesome departure with them having put a swamp in Hogwart.
It was really funny and awesome to see Umbridge and her goons being targered by spells, Nifflers and above all by Peeves who made life a living hell for Umbridge.