It's very, very hard to choose a favorite, so instead I'll talk about some of the things that stand out to me as favorite parts in the series: The entire whimsical, eager tone of adventure and discovery the first book has, the tonal shift and introduction of Sirius and Remus in PoA, and Dumbledore's Army in OotP were all really, really interesting reads, and the reason i draw myself back to those particular books more than the others. For nostalgia's sake, however, it's hands-down Chamber of Secrets, which was the first book I read in the series (it had gotten thrown out for growing mold by our librarian, and I didn't think it was right to throw away a book without reading it so I snatched it from the bin. I was just a tot at the time, and thought reading books from a trash can was perfectly acceptable.)