Your vote for greatest (covers) album

Abba-esque EP by Erasure really jumps first in my mind. Covering 4 ABBA songs and making it completely theirs. 

The Scissor Sisters glam, disco cover of Pink Floyd’s “Comfortably Numb” is so wrong...but yet so right

I don't know of any album sloppier than Sid Sings, but I, too, have enjoyed it a lot.
Yes, the Motorhead covers album is good, and I also liked Guns N' Roses' Spaghetti Incident and Metallica's Garage Days Revisited; both had a couple low points, but the highlights more than made up for it.In that same era, Slayer put out the note-worthy Undisputed Attitude, which had its moments and was mind-blowingly fast, even for Slayer.
My all-time favorite cover album, though, has to go to The Walkabouts' Satisfied Mind.  It is a phenomenal album by any measure.

Bryan Ferry's These Foolish Things (1973). Covers of the Beatles, Stones, Dylan ... I have the CD on order!

+1 mammothguy54! Bowie's Pinups is great! I forgot about it! Thanks for the reminder!