Your vote for greatest (covers) album

David Bowie is an album entirely of "cover" songs.  That alone makes it one of the best, if not THE best, for "covers".
My two cents on the matter.

And for those thinking of album cover art, definitely Yes albums and anything from Roger Dean.  Although S. Peppers Lonely Hearts Club Band is certainly a stand-out classic of album art unto itself.

I forgot to indicate which David Bowie album.  Sorry about that.
It's Pinups.

Individual artist:
Matthew Sweet & Susanna Hoffs - "Under The Covers, Volumes 1 - 3."

Compilations:  
"Heaven & Hell, Volume 1": A Tribute To The Velvet Underground

"Starman:: UNCUT Magazine's rare and exclusive versions of 18 David Bowie songs.

"Spirit of 73";  Good overall; but, the Rosanne Cash cover of "River" is much better than the original by Joni Mitchell. I did not think it was possible for anyone to outdo Joni on one of her own songs..

cassandra wilson adding lots of slink and sorrow to U2’s “love is blindness” and Van Morrison’s “Tupelo Honey”

+! on "I'm Your Fan - The Songs of Leonard Cohen."