Mine would have to be Prisoner of Azkaban. I love the dark humor and the quirky touches everywhere (the talking shrunken heads on the Knight Bus, Aunt Marge and her dreadful dog, so many more). I also really like the constant presence of clocks throughout and the complex way they interweave the plots from the different time streams (I know this is written into the books, but how they do it visually--like show the headsman's axe going down on poor Buckbeak and then, oh no, it's just a pumpkin!)