Favorite Book Cover
This is a tough one. I am American, so the book covers I am used to are the Scholastic ones. I believe they are tons better than the Bloomsbury ones, but you can disagree with me if you want. I do like some of the international covers as well. Check out The Leaky Galleries to see all covers.
As for my favorite, I'm not sure I have one, but I do enjoy this cover because it features Ron and Hermione on the back side: