Oooh....TOUGH TO CHOOSE! But that's probably because I am also a raving Harry Potter fan.
Well, first off, I am absolutely awed by the British adult covers - I never saw those versions before, just the children's versions. How amazingly different, and appropriate.
Of the rest, I enjoyed the Dutch and the German versions, for different reasons.
I like the Dutch series because the covers are very mysterious and some unique design decisions were made: you never see anyone's face, and with exception of the last cover, there is always a figure just leaving the page. Also, these covers featured the head of a different key character in the graphic at the top featuring the author, J.K. Rowling's name. It would make me wonder who or what that character is and why it is featured on the cover. (Never mind that they reverted to the American/English title design after the second book.)
After perusing through the entire site, checking out the unusual Dutch covers, the German covers really started to jump out at me...and make me laugh. I realized a duplicate Harry sits at the forefront of each cover. Behind this "forefront" Harry, the illustrations feature another Harry in key parts of each book. I found this arrangement a bit odd. And Harry doesn't seem to age much as the series of covers progresses. (In the stories, we follow him maturing from a naive, scrawny 11 year old to a selfless, responsible 17 year old.) This series of covers is by far the weirdest to me. (Never mind that they reverted to the American/English title design after the third book.)
