If I had to pick one it would be Marquee Moon. Overall a perfect package. Don't know what Mapplethorpe did to the image but that grainy blue hue surrounding the band, the black framing, typography, all just works to evoke what's inside. But the back cover is also great with that b/w spiral image and the photo inside of the band with Verlaine and Lloyd sitting on their chairs.
Some others would be:
MBV - Loveless
Sonic Youth - Daydream Nation.
Only Ones - Even Serpents Shine (Looks a bit naff now but still love it)
ISB - 5000 Spirits
Annette Peacock - I'm the One
Almost any Impulse sleeve but at a push Coltrane's Africa/Brass. Something heavy on his mind.
Miles - Bitches Brew
Ornette Coleman - Free Jazz
Kevin Drumm - s/t (Which i nicked for my avatar)