What is your favorite album cover?

Ambergris
Its different

https://img.discogs.com/opJ72FmAq09K7gKUQ4yrVdYEu4s=/fit-in/299x297/filters:strip_icc():format(jpeg)...

Deutsche Grammophon as a record label prefers fine art as subject matter for many of its pressings. Covers to numerous to include here.  
Casper David Friedrich and 
additional “Symbolist” artists are personally my favorites.....

Rush - Grace Under Pressure!

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