I’m partial to Weasels Ripped My Flesh.
I especially like the “RZZZZZ!” part.
Probably not a very pc response considering this is Easter weekend, but Camel’s I Can See Your House From Here has always been one of my favorites.
I’ve always been a Roger Dean fan, but one of my earliest memorys is of Herb Alpert’s Whipped Cream and some real creepy artwork on a copy of Sorcerer’s Apprentice.
The Pixie’s Trompe le Monde is disgustingly fascinating however. (Would someone in touch with the 20th century please link)
I’m gonna find a copy of Whipped Cream and leave it laying around to see how my 4 year old boy reacts to it… 
KISS Rock And Roll Over
The first Black Sabbath
Judas Priest Sad Wings Of Destiny
Motörhead Orgasmatron-I actually have this tattooed on my shoulderblade.
And too many by Iron Maiden to list, especially the singles
This pair of album covers always made me chuckle:
Whipped Cream and Other Delights, Herb Alpert and the Tijuana Brass
Clam Dip and Other Delights, Soul Asylum
I’m also a sucker for comics artists doing album covers. I believe this Miles Davis cover by Don Martin was among the first.
The Grand Illusion by Styx is good and trippy.
Or how about Hemipheres by Rush. A guy in a convervative english suit and a naked guy in a balet pose squaring off atop a huge brain repesenting the left-brain/right-brain dichotomy.
Someone else has linked, but I’d just like to point out that that’s one of the examples of V23/23-Envelope’s work that I really like.
“Stan Freberg with the Original Cast”. It’s a photo of Freberg and his foot is apparently broken because it’s encased in plaster.
Rush’s Hold Your Fire. It’s just perfect in its perfection.
U2’s All That You Can’t Leave Behind is a personal favorite, mainly because it matches the music of the album fantastically. It’s a simple, beautiful shot, just like the songs inside.
That was going to be my nomination. I had the LP, and even my CD version still has all the pages and text of the original.
I like the album covers for the British band The Beautiful South.
The cover of their first album drew criticism because it paired a picture of a guy smoking with a picture of a woman putting a gun in her mouth. In response, later printings of the cover had a teddy bear and a rabbit.
There was also some controversy over the cover of their later album Miaow. Apparently, HMV asked for the cover to be changed because it mocked their trademark dog and gramophone, so the old cover was replaced with a different one.
An artist named “Hip Gnosis” used to do Al Stewart’s album covers back in the 70s. He snuck images of Dr. Strange into two of them, Modern Times and Past, Present and Future.
Modern Times* was reissued with different artwork, though.
I kind of suprised that no one has mentioned Black Sabbaths ‘Sabbath Bloody Sabbath’ yet. It is probably my favorite cover.
I had many discussions about this cover back when I was a stoned teenager. Nothing like getting stoned* and debating what the Sabbath cover meant.
Slee
*Of coarse, we were stoned so the discussions weren’t exactly coherent. Man I am glad I stopped dong that kind of crap.
Hypgnosis was the working name for Storm Thorgerson and Aubury Powell, who were probably the premier cover artists ever. They did nearly all of Pink Floyd’s covers, and covers for Led Zeppelin, Peter Gabriel, Paul McCartney, 10cc, and many others. If it’s the album has a striking photographic image, there’s a good chance Hypgnosis was involved.
It’s worth mentioning that the late comedian Phil Hartman was once a commercial artist who designed album covers for the cream of the lightweight 70s bands and others back in the days of vinyl LPs (Poco, America, CSN, Firesign Theatre). None of these covers are anybody’s favorites, but hey, it’s Phil Hartman!