Alright, here's a few of mine...
Immortal - Battles In the North
Great album, one of Immortal's best, imo, but that cover is just plain stupid. If this was a simple black and white cover like the classic Darkthrone covers it wouldn't be nearly as bad, but that plain white background is terrible.
Monstrosity - Millennium
Musically, one of the better examples of technical death metal done well. That cover though..... not so much. Every time I look at it I'm baffled as to what I'm actually looking at. The artist who rendered it probably didn't know either lol.
Dawn - Slaughtersun
One of the best black metal albums ever, but I can't stand the cover. Just simply not a cool image, doesn't suit the music either. Visually there's too much orange and yellow and it honestly hurts my eyes haha. They could've done a lot better here.