Great Topic!
Best:
Def Lep: Hysteria, pyromania, retroactive
Metallica: Ride the lightning,master of puppets
Black Sabbath: Sabbath Bloody Sabbath
All iron maiden covers
Metalium: Millenium metal volume 1
Lynryd Skynyrd: The Last Rebel
Everything by Journey

Worst (good albums, bad covers)
Everything by the Marshall Tucker Band
Helloween: the best, the rest, the rare
Trixter: Hear!
Poison: Native Tongue
Halford: Ressurection (looks like they spent more time on the music than the cover,,not that im complaining)
Marvelous 3: Hey Album
All of starships covers


Bad Albums, Bad covers:
Steelheart: S/T
Metallica: S&M
Alice Cooper goes to hell
Queensryche: Hear in the now frontier
Pearl Jam: Ten (except for alive)

Good Covers, Bad Albums:
King Kobra III
Rush: Presto
Shogun: 31 days (the covers good for a laugh at least)