Harry Potter and the Half-Blooded Prince Discussion

Just finished reading the sixth installment of the Harry Potter books. I refused to go to sleep until I got it done. Anyway, I just wanted to throw a question out there for discussion. Which Harry Potter book do you think is the best? Why?

My own resolve lies with book 5. I think the story is the most developed and layered with character development. I also have to admit that the more difficult life gets for Harry, the more the story captivates me. Book 5 definitely doesn't hold back in life lessons and difficulty for Harry. I think also at this point we are all interested to see what the characters will be fleshed out as in the movies. This book has set the stage for the most interesting cinematic possibilities. Okay, friends, what do you think? Book 1-6? What's the best Ms. Rowling has produced?