For mine...
Goblet of Fire (novel): The silly subplot of Hermione trying to form a group fighting for elf rights, about a day after finding out that they are secretly being enslaved around the school. Despite no one agreeing to join S.P.E.W., & her being told again & again that the elves are proud of their jobs, Hermione just refuses to give up. I'm glad this wasn't included in the film, because it became very ridiculous after a while.