POA. It's the book that gave us Remus and Sirius, for one. Also, to my mind it's the best-constructed novel of the series. It's got a very complex plot (especially that time turner stuff) but it's never draggy or bloated (I'm looking at you, camping chapters in book 7). Also, I think I like it best in part because Voldemort's not in it. Some of the later books are pretty damn frightening in a horror-genre way (as opposed to a political allegory way) and I have trouble with horror.

Also, POA the book gave us POA the movie, and without movie!POA I would not even be here in fandom at all.