I can't even say. I thought the Goblet of Fire is a great, suspenseful novel with brilliant action sequences and the point where the series can no longer be called children's literature. At the same time, Order of the Phoenix is really well written and has the best character of them all, they both pick up in each others minor flaws to the point that their perfect and tied in my eyes. It's like deciding between Alien and Aliens.
Deathly Hallows because it was satisfying end, second favorite would be Chamber of Secrets because it had eveything in a good fantasy and had all my favorite scenes in the book: the flying car, Dobby, Gilderoy Lockhart, The Polyjuice Potion, The Basilisk, the spiders, and the Dueling Club. That book besides GOF and DH are the most cinematic of the books because its a high flying adventure and the movie is on my top ten favorite films of all time for me.