Best album covers. IMHO these are interesting and in no particular order

1.Gamma 1-The transparent man cover always looked great. Mick Haggerty
was/is an awesome graphic artist, and I last heard he turned to video
directing.
2.Black Sabbath-Live Evil. Stan Watts did outstanding work.
He also did the "Metal Health" album by Quiet Riot.
3.Most seventies and early eighties stuff by Hipgnosis.
(Bands like Tesla, Scanner, and Helloween, Dream Theatre tried to copy the artwork feel of Hipgnosis but came up short to the originals)
4.Van Halen 1. This album cover was simple but many bands
copied the same layout-Steeler "ST", War and Peace "Time Capsule"
Lancia "ST", and a few others.

Worst

1.Most everything done in house on Metal Blade, the old
Neat Records, alot of Long Island Records releases,
certain MTM records, the recent spate of CG album
covers that all look the same on minor AOR labels,
the old Megaforce/Johnny Z label out of New Jersey,
most metal indie labels, the first Shrapnel records,
etc.

Colour Crayon awards go to

Marcie Free-Tormented on Long Island. (Hard to believe the label
gave the okay to put out this cover, and Marcie Free must've been
horrified!)
Born to Metalize-Megafarce Records. This one had to be seen to be
believed. Pure elementary school large paintbrush/crayon rendering.
Hawaii-One Nation Underground-Shrapnel Records. Another one of
those "used a large broad paintbrush and water color paints" jobs.
Seriously laughable. If I was in the band, I would've sued Mike Varney
for defamation of character.
24K-Pure. The Japan release of this looks pretty classy and put together
well, while the Z Records version looks like somebody took their lunch
hour and pasted a cover together.

There are many more horrendous artwork jobs out there.