I love them all, absolutely, but I think if I had to choose my 2 favorites, it would be The Half-Blood Prince (My favorite Harry Potter book in the original format of a mystery at Hogwarts that Harry and friends have to solve) and the Deathly Hallows, which followed a completely different format and took our protagonists completely out of the comfortable fortress of Hogwarts and into the wilderness, where they fought a very adult battle with very little help and support from other adults for the first time. All books are masterfully written, but in these Harry and his friends really begin to come into their own and we get to see the true complexity, darkness, and grey areas of the wizarding world and all its secrets, which come to light more and more as they are forced to grow old enough to grapple with them.