I've got a different one there bacobit... only this time it's not nirvana! :P

But seriously now... some of my favorite covers are these:
Queen - The Miracle
This cover represents (for me) how closely intwined the four members of Queen were, not only on a musical level, but on a personal level as well... shame the picture 'fell apart' after Freddy died... :(

Van Halen - Balance
This album cover shows a great visual presentation of the album's name, both the 'bad/angry/wild' and the 'good/friendly/calm' sides are represented in it, just like the type of songs on this album are!

Can't think of any more right now, but I'll check in again when I come up with them! ;)