As you know, I’m a big ELO fan. However, their album covers leave much to be desired.
By far the worst is On the Third Day followed by Balance of Power.
They are best known for Out of the Blue, which gave us the famous spaceship used in their 1977 tour.
Somehow, the Borg got a hold of it in 2001 as can be seen on the cover of Zoom.
I guess their best cover would be Face the Music or Secret Messages.
The Beatles
Best: Revolver
Worst: Magical Mystery Tour
Bonzo Dog Band
Best: Tadpoles (the original die-cut version
Worst: Keynsham
Traffic
Best: The Low Spark of High Heeled Boyw
Worst: The British version of “Mr. Fantasy.” The US version is OK, but the Brit is just an ugly red color.
Rush!
Best: Moving Pictures - great because of the triple play on words. Front cover: Men in coveralls are carrying paintings into a museum - moving pictures. A family is watching the proceedings, in tears, because the pictures are so moving. Back cover: A film crew is recording the proceedings, making a moving picture.
Worst: Grace Under Pressure. I’ve never quite figured out what I’m looking at.
Genesis:
Best:
Hard to say. I’ll go with Selling England by the Pound. Nursery Cryme is also up there.
Worst:
I’m not counting the numerous rereleases of From Genesis to Revelation. I’ll say Invisible Touch has a fairly blah cover.
My favourite band: Blur
Favourite album cover: Parklife, followed closely by Modern Life Is Rubbish and 13.
Least favourite: Blur (self titled) or The Great Escape. I have to wonder if my negative reaction to these albums influenced my dislike of the cover art, though that would be weird. I think it’s just a coincidence.
Their first album, Leisure, with this cover art, is okay.
My favourite CD single artwork was for Chemical World - a hard-earned and expensive rare I procured shortly before buying the damned Anniversary Collection, which had the single with original album cover. :smack:
At least I had the cool… uh… jewelcase. Fine, I had nothing, I blew $60 USD on five songs I would later have anyway. 
(three of which I already owned… yeah, I used to be pretty die-hard.)
Pink Floyd
Best: Animals. (WYWH comes in a close second.)
Worst: hmmm, I’ll have to go with Obscured by Clouds
Queen.
Best album cover: News of the World
Worst album cover: Hot Space
It’s hard to single out one favorite band, but two strong candidates for that honor are The Kinks and Daniel Amos.
The Kinks: Alas, judged strictly by their album covers, they just don’t rate very highly. Too many of their covers are uninspired, inappropriate (e.g. the cover for Face to Face, which is far more psychedelic than anything on the album), or downright ugly. The ugliest, which probably gets my vote for lowest cover-quality-to-album-quality ratio in all of rock, is for Lola versus Powerman and the Money-Go-Round, Part One.
On the other hand, I like the cover of Phobia and am perversely fond of Schoolboys in Disgrace
I like the cover designs for Daniel Amos (a.k.a. DA)'s Motor Cycle and Kalhoun. Not as much the “naked man doing sit-ups” cover image for Fearful Symmetry.
The Incredible String Band, naturally.
Best: 5000 Spirits or Layers of the Onion. This is the album cover by which all other 60s era psychedelic album covers should be judged.
Worst: No Ruinous Feud. It sounds pretty much the way it looks, too. 
In order of personal preference:
Afghan Whigs
Best:Black Love
Worst:1965
I entered this thread specifically to enter this worst–agree totally (I probably agree with your best, though I can’t forget Sgt. Pepper, obvious choice though it may be).
In fact, as a side comment, I would wager that this album is unsurpassed in the differential between how good the music is (pop masterpiece) and how bad the cover is (garish doesn’t begin to describe it). Go on, I defy you. Find a bigger differential. Here are the songs just to remind you:
Pet Sounds is close. The most prominent cover feature is a herd animal’s ass. The album cover looks like my dad posed them and shot the photo. What an, um, unspecial album cover for such a musical treasure.
Anyway, carry on.
I love Paul Westerberg, but the cover of his Come Feel Me Tremble album is garbage. The image of him is fuzzy, as if it were a still from a cheap movie camera. I probably would not have minded that too much taken by itself, but the cover is also marred by a bright white square containing the title that blots out the lower half of his face. It would have taken no effort at all to move that square over to the other side of the cover for something far more aesthetically pleasing. The whole effect screams, “I couldn’t give two fucking seconds of thought to creating a halfway decent cover.”
The music, on the other hand, is superb; it’s easily my favorite one of his solo records, and compares favorably with Westerberg’s work with the Replacements.
Belle and Sebastian
LPs:
Best: Legal Man
Worst: Push Barman to Open Old Wounds (worst title, too - and B&S have had some doozies)
Well, that’s screwed up. “LPs” isn’t supposed to be there - Legal Man is actually an EP.
I’m a little bit of a band-killer – as soon as I get interested in a band, they break up. So most of my favorites don’t have enough album covers to make a list.
Soul Coughing
best: Ruby Vroom
worst: Irresistible Bliss
The Pogues
best: Peace and Love
worst: Red Roses for Me
Pizzicato Five
best: Voyage a Tokyo
worst: Made in USA
some favorite '70s-era bands
**Blue Oyster Cult ** (tough- most of their covers are pretty memorable)
best- Fire of Unknown Origin
worst- Club Ninja
Cheap Trick
best- Dream Police
worst- The Doctor
Kiss
best- Love Gun
worst- Asylum
The Who
Best: Who’s Next
Worst: Magic Bus
Led Zeppelin
Best: Physical Graffiti
Worst: Led Zeppelin II
