I'm not quite sure whether I'm supposed to pick my favourite cover from among the books or in general, but I'm going to do the latter. I am half-German and it was an aunt on the German side who got me my first Harry Potter, which naturally means it was in German. I think each translation also got its own cover art, so I grew up with the following covers:
I absolutely loved them and still do. I feel all of them captured some of the best scenes in the book without giving away too much and I think they kind of draw the reader in as well. And I just love the cocky eyebrow that Harry is sporting. This drawing really fits well with the sarcasm we often get from Harry that wasn't in the films as much. And the way he pops up also kind of makes it look like a detective. Give the boy a pipe and a coat and he could pass for a young Sherlock, no? Out of these covers, Order of the Phoenix is definitely my favourite!
So, what does your favourite cover or edition look like?