Best-
Rolling Stones-Some Girls (original cover)
Led Zeppelin III
All the Boston covers
Van Halen I
Van Halen - 1984
Van Hagar - 5150
Van Hagar - Live Right Here Right Now
Beatles - Sgt Pepper
U2- WAR
Ramones - Road to Ruin

Worst- (good albums, bad covers)
Sex Pistols - Never Mind the Bollocks (especially the American CD reissue- Great album, lame cover)
Operation Ivy - Could have done that on a Xerox machine
First 2 Green Day albums - ditto (no pun intended)

AC/DC- Back in Black
Metallica - Metallica
Prince - The Black Album
Spinal Tap - soundtrack (try telling these four album covers apart when you're drunk)

Beatles - White Album (had to have been Yoko's idea)
AC/DC Live - who the fuck designed a CD package that you had to store with your vinyl because it won't fit in a CD crate??
(honorable mention to Pearl Jam's Vitalogy, but at least it fits sideways)