I would have to choose Harry Potter and the Prisoner of Azkaban. I've said this on another answer, but I like it because the plot is very different from all the others, turning away from Voldemort. It's the only book where Harry doesn't come face-to-face with Voldemort in one form or another. Because the book doesn't focus on Voldemort, it gives Ron and Hermione a bit more room to shine. I like this, because they, especially Ron, seem to get pushed aside a bit in the movies.
Also, Hermione is my favorite character, so I like it how, in both the book and the movie, Ron and Hermione do a lot more than in the others. Especially in the Goblet of Fire, Ron and Hermione hardly do anything special. Ron argues with Harry, Hermione sort of gets a boyfriend, and they both help Harry with the tasks, but other than that they don't really do much.
In the Prisoner of Azkaban, all three of the main characters grow and develop a lot more as they are now teenagers. Their personalities get more distinct, and the relationships that they have with each other grow stronger.
The climax of the book was extremely exciting (it never gets boring) and I love the way they adapted it for the screen.
I'd have to say, after the third book, I liked the Order of the Phoenix, followed by the Goblet of Fire, the Half-Blood Prince, the Sorcerers Stone, and I think the Chamber of Secrets was my least favorite, NOT to say I didn't like it. All of the books and movies are AWESOME!!!!