Another tricky question. For years, as we all know quite well, DWJ covers were mostly terrible. Finding ones I like has been difficult. These days the covers are far better, though.
I love the two different sets of covers that the Dalemark Quartet has had in the last several years. The cover of Crown of Dalemark is awfully pink but otherwise they are great. In the second set, I love how different pieces of the map are used as background with a sort of emblem superimposed on top. Very nice.
My copy of Power of Three is from the UK, and I've always really liked the cover, but it was hard to find an image. This one is tiny.
I have to say this cover of Time of the Ghost is quite effective. I don't have it but I would like to.