My favorite Harry Potter book is Deathly Hallows because the trio has to save themselves without Dumbledore or anyone else in the falculty's help :) The book really shows their strengths and how much they've grown since the first book!
My favorite book is Harry Potter and the Half-Blood Prince, but I admit my reason is a bit shallow. It mostly centers around Draco Malfoy, my favorite character, and out of all the Harry Potter books this is the one where he gets mentioned the most. I love his development from his older, even more arrogant character from the beginning of the book to the nervous wreck at the end (yes, I'm one of those fans). I also love all the light-hearted comical dating relationships throughout the book and how we got to learn about Voldemort's and his mother's past.
The Order of the Phoenix is my favourite, because:
2) The Weasleys Twin's Great Escape
3) The DA
4) The fight in the Ministry
5) The undertone of rebellion in the book
6) Luna Lovegood
7) After a few minutes of remembering this book, I now want to read it again!
While I obviously love all the HP books to a more or lesser extent I think my number one favourite (if I had to choose) would be The Order of The Phoenix. While other books addressed the issue of Inevitability of a fight when it comes to defeating evil, this book not only made it more bold but made it much more real because it introduced the different kinds of evil that could be equally harmful (evil not just in a form of Voldemort, Death Eaters or some scary creature Basilisk, but also in a form of a smily lady who loves cats and also enjoys sadistically torturing her students by forcing them to write "I must not tell lies" that reflects on their hands with bleeding lines), it was about facing the unfair situation whereyou might end up being labeled as evil too and will have to choose between proving that you are not a liar and between surrounding yourself with people who would believe you anyway without asking for any proof because they trust you. It was about rebellion, dealing with aftermath of death (Cedric), Harry enduring another death of a person whom he considered a family and the impact it had on him (his parents' deaths obviously influented on his life in a negative way, but he never got to directly react to their murder because he was too young back then and couldn't process it so we mostly saw how he handled being an orphan but not how he handled losing them. And in Sirius' case we got to witness all the heartbreaking nature of a loss through Harry's pain). This book combined emotional, political and philosophical aspects of life and JK Rowling, unlike many writers that don't manage to pull it off convincingly ending up making such combination contrived and over the top, did a great job with showing how all those issues effect people on personal level.
And on a shallow note: Luna Lovegood. Enough said. And Fred & George Weasley rocking it in the end.
my favourite of the harry potter books has to be the first one, as it is the total beginning for everything that follows from a basilisk to voldemort himself, and it is the perfect way to start such an amazing series, especially the introduction to the wizarding world- diagon alley! and it goes into depth about harry's life and how he adapts to wizard life.