Harry Potter and the Prisoner of Azkaban
Book three is probably my second least favorite in the whole series. However, the first few chapters and the first 10 minutes of the movie really make me laugh. Aunt Marge blowing up is one of my favorite scenes out of all the movies.
I don't have a lot of specific reasons why I put this book so low on the list of the series, but I think a big part of it is that I have so many favorite parts in the other books and this one just doesn't have as many favorite passages. My list just isn't as long.
Boggarts (specifically Hermione's), Harry finally being able to see his Partronus, the Quidditch Final and the Marauder's map talking back to Snape are my favorite parts.
Book four is probably my second favorite - so book three always brings the excitement of knowing book four is next!