My absolute favorite book is Prisoner of Azkaban, because, like someone else said, of the mystery plotting and also because of the Marauders' backstory. My favorite character is Sirius, and I also like Lupin and Harry.
I like the movies a little, but I think they pale by far in comparison with the books. 5 was my fave movie because I actually thought they did a really good job of paring down the story, but 4 was absolutely horrible (except for David Tennant, but that's a whole 'nother thread).