I've only read the first two.
So I'll say Chamber of Secrets.

Hallows though I'm sad ta see the series end

HBP. First one that I had personally, and my ex-stepmom got to bring the SIGNED COPY OF IT TO THE LIBRARY WHERE SHE WORKED.

Though, Order of the Phoenix is up there too.

But really, it's all of them. They're all one big long book in my head.

Well, I've read them all so many times that they all blend together for me. Usually when I go through them now I just skip all the battle shit and focus on the social/class stuff, because I find that the most entertaining: average life of a Hogwarts student and all.

But if I really have to choose, I'm probably going to say either the first one, or Prisoner of Azkaban. I especially liked the part where he was living on Diagon Alley for a week.