I'm not sure since it's been so long since I've read the books but maybe Chamber of Secrets? I like Prisoner and Azkaban and Goblet of Fire a lot too, but I have a soft spot for Chamber of Secrets because it was the first book I'd ever read that truly freaked me out. The idea of monstrous snakes in the plumbing scared the crap out of me for at least a week after I'd actually read the book, trufax.