*discovers thread*
Well, even if the poll gets restarted now, I voted for Philosopher's Stone. It is the question of all questions though, which is the best!
Book four is my least favourite of all five of them but I'm not really sure why. Perhaps it was the lack of Quidditch (apart from the world cup of course!). I like PS because everything's so new and we're just discovering the wizarding world, although saying that, I like how comfortable magic has become in the later ones - partly why I love OOTP. It's a tricky one!