Prisoner of Azkaban is my favorite out of the whole series. It brings in so many different magic elements that haven't been present before, and I find it really interesting that this is the one story where Voldemort is not directly connected to. The closest we get is Trelawney's prediction about Wormtail returning to his master. I absolutely loved the different creatures mentioned in both Care of Magical Creatures and in Defense Against The Dark Arts, and I love that we learned about Animagi. The Marauder's Map is also bad ass, and I wish I had half the imagination that J.K. Rowling does.