I was about to write that my absolute favourite is MM, but local hero got there first.It is plain like the Beatles white and it both visually and commercially works, since it sticks in your head. And the combination of colours!!!
I think that the cover is as much important as the music contained, as far as the commercial part. A fine or intriguing cover, will attract more customers, at least to listen to the music contained. It shows that the artists care for every detail of his work. On the other hand, sometimes the music is so good or the band so famous (The Beatles "white album" or any successful album that bares just a photo of the artist) that the cover is of little importance. Of course it is a double mystery that we can not solve since the extremely commercial albums tend to enter the public conscience even if their cover sucks. Dylan's covers have done the impossible, most of them have him on the cover, but they transcend the photo of the musician, into a photo of the times and events. We actually see a hero (that grows with us) that represents what we wanted him to be. This is not done only by the photographer/artist or the treatment of the photography, but also from the mythology weaved around Dylan as well as the type of songs contained and the cultural intensity of Dylan's music. I believe that only 3-4 albums don't have Dylan on the cover. But that stood for vinyl, and that is why I used to buy LPs for ridiculous prices, just for their covers! Take a look at those:
http://rateyourmusic.com/list/monocle/look_closer__visual_tricks_and_hidden_images_in_album_coversAs far as MK's future albums, I really don't care much about the covers. I can't imagine something that is not made, how it would be. I guess the fixation with MK is bigger than the cover. I don't like GH, I find interesting the STP (I took such photos myself and was pretty surprised to see a similar one becoming the cover art of my fav musician)
TRPD is a great photo that suits the mood of the album perfectly, ATTR is nice, SL is OK but nothing more, and so is GL. I like KTGC but as in TRPD it is the art that works regardless of the album. Oh and BiA must be one of the worst ones but I secretly like maybe because it has entered the public conscience. Oh and Alchemy, is the one after MM for me.