For me it was (in no particular order):
Led Zeppelin Physical Graffitti (jeez...it's got "windows")
The Beatles Sgt. Pepper's (stared at it for what seemed hours. In a way, the cover was kinda creepy after all that "Paul is dead" and the hand over his head ("number 9 number 9") stuff that later unfolded.
Osibisa Wcyaya (elephant on steriods, Roger Dean, need I say more?)
Yes Relayer (Dean again. Without a doubt all time fav album cover artist.)
Traffic The low spark of high heeled boys (oddest shape, with those angled cut corners.)
Parliament Motor Booty Affair (wow...it came with a comic book, pop ups, and everything. Y'all don't know what yer missin' with this CD s#it.)
Janis Joplin Cheap Thrills (speaking of comix...)
R. Wakeman Myths and Legends of King Arthur...Round Table (the cover wasn't worth a damn. It was the interior booklet with little writeups of the legendary Arthurian characters that made for fun stuff.)