I'm really surprised that I'm the only one who's voted GoF. I hated PoA for the longest time, and after a few rereads as an older reader, I like it, but I've always loved GoF. It's the middle book, the transition. I think it marks the point where Harry learns he really has to come to terms with the world, because Voldemort's back. It's also the first time Ron's loyalty to Harry is really tested, and it's so sweet.
"My pet, I've been to the devil, and he's a very dull fellow. I won't go there again, even for you..."
"Harry Potter is about confronting fears, finding inner strength and doing what is right in the face of adversity. Twilight is about how important it is to have a boyfriend." -Stephen King
For in everything it is no easy task to find the middle ... anyone can get angry—that is easy—or give or spend money; but to do this to the right person, to the right extent, at the right time, with the right motive, and in the right way, that is not for everyone, nor is it easy; wherefore goodness is both rare and laudable and noble. — Aristotle