No question, Tull's Thick As A Brick.
I have a first pressing Reprise MS-2072, in mint condition, and it's awesome. That has to be the most creative album artwork ever. I've never seen anything that compared to it.
This is a long thread, and being lazy I didn't bother to check and see if it's been mentioned before, but Roger Dean's stuff always struck me as the acme of what the album cover should be.