Harry Potter and the Prisoner of Azkaban
I can't decide. But, hear this: I love every book in the series but I have to pick this one because this is the first one I've read and I really knew this book by heart. But honestly, every book is great. I'm actually torn between POA and DH. But Merlin's beard! HBP is so funny and OOTP so action-packed and the fact that PS started it all or because of COS' humor or because they're all grown up at GOF.... This is really a hard decision. But still, I really liked POA.