Prisoner of Azkaban was the first book I ever read and I think for that reason, it's always been my favorite. It was the book that introduced me to the world of Harry Potter, and thus became my gateway into the entire fandom. If I'd never bothered to read it (or rather had anything else to read - I was 11 years old and had already read all of the books in my AR level at school, so PoA was it, hah!) then I never would've found the same community that made me who I am today. Not to mention there were werewolves, hippogryffs, animagi, and all sorts of cool stuff in book 3. How can you NOT like it?

(Also Hermione smacks Draco and that was pretty much the biggest highlight for me outside of Sirius Black's storyline, haha!)