I'd have to say, concept wise and the way it was written etc would have to be the Chamber Of Secrets. That was when it started to become more intriguing and we got to know more about voldemort. And come on, 'kill kill, let me rip you...' who does not like that? I found it so chilling and eerie and it really was exciting to uncover the mystery. The other books kind of fluctuate, some parts are interesting some aren't. And the last 2 books god they're so depressing. I just don't like reading them they make me cry, I prefer the first three or four cos harry still is in the dark about the future and everyone is still happy and alive.