Actually, i reckon 'Opeth - blackwater park' is the best cover art ever created... but of course, blackwater is not a nevermore release, so we shall just leave it at that.
I vote dhiadw as the best cover because I think it is very depressive and it hides many words inside. If you look carefully you will find out many messages of the world and the people !!!
All NEVERMORE covers are quite interesting. Even though all covers are considered art, I feel the most time, effort, and detail was put into Dead heart.... I also am a fan of Travis's work and really like the covers to Control Denied's "Fragile Art Of Existence" and Skinlab's "Disembody: The New Flesh. I'm possibly thinking of getting another tattoo, (I have 3) and incorporating some of the work from Dead Heart.. but not blatantly ripping it off. Check out this quote I saw on bumper sticker: