What is your favorite album cover?

Art is very subjective. Jazz music, in my opinion, had the best album covers. Many will say the Blue Note covers (Francis Wolff photography) but even before that there were the amazing David Stone Martin works for Clef, Verve, and Pablo. Also Andy Warholl's work on three Kenny Burrell's covers are classics. Some of the CTI covers have nice photography as well. For more modern works:
War-The World Is A Ghetto
Jimi Hendrix-Electric Ladyland (the version with the girls on the cover)
Santana-Santana
Cheap Thrills-Big Brother and the Holding Company (Janis Joplin)

Houses of the Holy - Led Zeppelin
Yellow Submarine - The Beatles
Duke - Genesis
Year of the Cat - Al Stewart
Draw the Line - Aerosmith
Boston - Boston
Who’s Next - The Who

Beatles For Sale on Parlophone. They were too cool for the time.

Small Faces' Ogdens' Nut Gone Flake

Led Zeppelin IV