Another honorable mention has to go to Led Zeppelin for In Through The Out Door. The album came with six different covers, each a shot from the perspective of the different people in the bar. Cool enough, but the real clincher is on the inner sleeve, which was a pen and ink drawing of a place at the bar, ashtray, cigar, bottle, lighter, cash and coins, lemon drops, etc. If you wet your fingertip and touched the individual items, the y changed to an appropriate color--the lemon drops became yellow, the cash green, the bottle blue, the lighter gray, the ashtray kind of blue-green, etc. As far as I know, it was only on the original issue of the album; subsequent pressings did not have this feature. A few months ago, a friend bought a copy from a used record store. We tried it out and the watercolor paint still worked. Pretty cool gimmick.
Also, the Grateful Dead get points for hiding the words "We Trip" on their first album, the word "acid" on Live Dead, and the words "we ate the acid" on Aoxomoxoa. Also, the cover of Blues For Allah, with the grim reaper playing the violin is major cool artwork.
Another cool one is sthe Rolling Stones' Their Satanic Majesty's Request, with the moving picture of the band on the outside, one of those things like you'd find in crackerjacks or something, where you get the illusion of motion when you tilt the oicture right or left. The inside artwork is pretty cool, too.
The woman on the cover of the Pixies' Surfer Rosa has an incredible pair of tits.
I always liked the cover art on Heaven and Hell by Black Sabbath, just for the concept, the fallen angels smoking cigarettes and playing cards, waiting on judgement day.