Harry Potter and the Order of the Phoenix. It's my favorite because it really shows a lot of conflict. Half of the Wizarding World dosen't believe Harry, while a small part of the population does. And while the parents (and the kids, but mostly the parents) are worrying sick about Voldemort's return, the kids have their own mini battle with Umbridge. It's a very intresting plot, and I love it.
probably the goblet of fire because its the longest lol and theres lots of action plus its the one where voldemort returns so the real conflict starts.
This is very hard to pick. I love all of the books. I think I should go with DH. It is the most emotional book in the series. You watch as the wizarding world goes into complete chaos when the Ministry of Magic falls. You watch/read how the story plays out. How the characters reacted. The pieces of the of the puzzle finally unfold.
Harry Potter and the Deathly Hallows. thats when it all comes together, questions are answered, mysteries are solved, there is an epic battle, and its definately the most moving IMO. I ball my eyes out every time I read it.