For a long, long time this was my favorite book of the series. I just loved all of the different facets of the story, and how dark it starts getting and how it gets really real and ominous. But now, Deathly Hallows is my favorite, closely followed by Order of the Phoenix, and then Goblet.
These books are just so brilliant, and Stephen Fry is so awesome at reading them. He's the perfect kind of reader to me, someone who can give characters their own voices and accents and breathe them to life without making them feel like caricatures and without interpreting them in such a way that it's counter to my own interpretation.