I've always been intrigued by MAYH. If I had seen that cover and not known what band it was, I think I would have picked it up anyway. It's so goddamn mysterious!!!! They'll never top it. It suits the album so well.
Morningrise is a close second....
And, just to note, I personally dislike their Travis Smith Covers somewhat. I mean, Travis is one of the best album designers in existence, and his work is incredible, but I feel that the artwork makes Opeth seem too ordinary. As soon as I saw the cover for Still Life, I was blown away, but at the same time disappointed. Somehow, a drawing lacks the realism of an actual photograph. The first three album covers speak loudly, yet show a more realistic side of things, more close to human. But the newer covers seem to be more, well, created, for lack of better word. Opeth is meant to seem natural, not sculpted by man. (Though after hearing BWP, I may think that the music is tending to follow the Covers......)