If uniqueness is the criterion, how about Monty Python’s Matching Tie & handerchief?
Emerson, Lake & Palmer
Note that the image is only half of the cover and that the LP version is far superior to the CD.
Also their Tarkus.
Also, as previously mentioned, King Crimson, In The Hall Of The Crimson King.
Why am I not surprised? 
Seconding these.
My choices:
Eugene Ormandy/Philadelphia Orchestra: Pictures at an Exhibition (LP only)
John Abercrombie: Gateway
Rare Bird: Rare Bird (US cover)
Jimi Hendrix: Electric Ladyland (US cover)
King Crimson: Islands (US cover)
Abbey Road
Sticky Fingers
Layla
The Wall
Breakfast In America
They all happen to be fantastic albums too.
I was going to go with this iconic cover, but I thought maybe I’d better stay inside my comfort zone. 
Time Further Out, Dave Brubeck Quartet
A Tribute to Jack Johnson, Miles Davis
Isn’t that the one that changes color if you wet it?
I’d never heard that. It did originally come in a brown outer sleeve to cover the fact that the photographs for the cover were from the interior of the bar from several different angles resulting in several possible different album covers: your cover might not be the same as the one your buddy bought.
Wikipedia confirms that - I had never heard of this previously.
That was the inner, inner sleeve. There were three sleeves - the outer brown paper bag, the regular sleeve with bar scene where there were six different versions from the six different people in the bar, then the inner sleeve which looked like a black ink drawing but was actually what we called “magic painting” as kids - a wet paintbrush released colour.
5 More
Night Ranger - Feeding off the Mojo
Pink Floyd - The Division Bell
Van Halen - Fair Warning
Jon Anderson - Olias of Sunhillow
Joe Walsh - Barnstorm
Only the inner sleeve. I wetted one of mine, just to see. I think if you go too much, the ink will smear.
Wikipedia says the color changing sleeve was never advertised. I don’t know where I heard about it. I figured the cover is a “clue” - the wipe across the cover brings out the color, just like it will on the inner sleeve.
I like that the different perspectives, even from the same person’s POV, are not the same. There are small changes in each of the 12 images. I learned about the code letter, and peeled back a few corners in the store, to try to make sure I got all six. Still had to get one on ebay just a few years ago. Surprising that many sellers didn’t identify which cover they had - I guess it wasn’t commonly known that there were 6.
A page about the covers : http://zeppelincollectables.com/pages.php?pageid=3
And, I still want to know what “Fully Coot” means!
That is a good one, isn’t it?
Classic.
Classic.
Brilliant!
What’s not to like about that? And the Roxy Music, of course. And whipped cream. OK, you get it.
Classic
Classic, along with others you mention.
Yeah, no kidding!
I see what they did there.
One of my all-time favorites, inside and out.
I have that on CD and never noticed it. It is really good, isn’t it? I guess size matters.
Gotta love it.
Thread winner!
Classic
And now I can’t remember my favorites, except perhaps this very simple but appropriate one:
The Police - Ghost in the Machine
As a kid I loved all the Yes album covers and art, but they don’t do much for me any more.
I like the artwork that makes you look again…
Bon Jovi: Slippery When Wet
Jackson Browne: Looking East
Cheap Trick: Dream Police (Inside and back covers were better; they were hilarious! Unfortunately, I can’t find links.)
Def Lepard: Retro Active
Styx: Grand Illusion
–G!
Without a doubt, my all-time favorite album cover: Camel’s I Can See Your House From Here
Dio, Holy Diver.
Top Five:
Zappa: Hot Rats
*http://musiikimaku.files.wordpress.com/2012/01/frankzappahotrats.jpg
*Doors: **Strange Days
***http://www.alexandruvita.com/blog/wp-content/uploads/2010/05/The-Doors-Strange-Days.jpg
*The Nice: Elegy
*http://eil.com/images/main/The+Nice+-+Elegy+-+Pink+Scroll+Label+-+EX+-+LP+RECORD-460885.jpg
*Zappa: One Size Fits All (BACK cover of the original LP- Cal Schenkel’s masterpiece)
*http://www.soundstation.dk/images/products/large/43/113743-b.jpg
*Jefferson Hairpin: Bless Its Pointed Little Head
*http://vol12.music-bazaar.com/album-images/vol12/544/544144/2396248-big/Bless-Its-Pointed-Little-Head-cover.jpg
*
Grace Jones - Slave to the Rhythm
Sly & The Family Stone - There’s a Riot Going On
Miles Davis - Bitches Brew
Madness - One Step Beyond
The Rolling Stones - Tattoo You
Great one; I almost listed this. Did you know that a gatefold version exists? The interior shows a rocky desert landscape strewn with photos and memorabilia of the Nice.