What is your favorite album cover?

Brain Salad Surgery & Ambrosia's Road Island. It's a Ralph Steadman drawing.  
Court of the Crimson King is music room wall art. One of the best.

Dark Side of the Moon

Jethro Tull - Stand Up is a good one, also Roxy Music - For Your Pleasure as well as the first eponymous release. I also like the cover of Blue Skies by Stan Getz.

Blonde on Blonde, one of the first rock gatefolds.
It really used the space with a fold out half-length photo of the artist in a suede jacket, the image slightly out of focus, seemingly alluding to the complex instrumentation of the music inside.
This iconic 1966 image rock-star image gave way a couple of years later to the backwoodsman image, still in the same jacket.
Happy 80th Bob.

The Rolling Stones ‎– Their Satanic Majesties Request