I wish I could pick a moment that defines each book for me. I totally broke down when Dobby died in the 7th and Rowling writes about how for Harry, it didn't matter any of the difference between them, he was losing a friend.
I also agree with Jaykumar about Snape in the 7th because it's the real upsetter; you never expected it and I feel it really provided the true essence of Snape in a single scene.
Then, of course, Alya makes a great point with her quote about Harry and his time in the Forbidden Forest, thinking about his friends. I do love the scene following with his parents and Sirius and how they say they'll stay with him until the end.
Too good. Too freakin good.