To me, if the cover is like the original recording, then there's no point in doing the cover. Hence, covers that suck to me include whoever redid the Monkee's song "Now I'm A Beleiver" for the movie Shreck. Actually, it's a Neil Diamond song because he wrote it and The Monkees only made a record that happened to be a hit.
Same for Uncle Cracker (?) remake of "Drift Away"- other than the bass is dull and lifeless compared to Dobie Gray's version, it's the same record. Pointless drivel, totally pointless. Add in whoever redid "Last Kiss" (was that Pearl Jam), Mick Jagger's remake of Bill Withers' "Use Me", Travis Tritt's "Take It Easy" (even if he DID get The Egos back together to sing on it), etc. And the worst cover of all was Rare Earth's lame and utterly unfunky rendition of "Get Ready".
Other just awful things are the way Nazareth totally ruined "Love Hurts"- As Emmylou Harris said "Love hurts, but the song shouldn't". Almost all of the "Sgt. Pepper" soundtrack comes to mind as bad covers. The only two that worked at all were Earth, Wind, & Fire's approach to "Got To Get You Into My Life" and Aerosmith's "Come Together" (I can forgive Joey Kramer's heavy handed and almost grooveless drums).
GOOD covers bring something totally new to the song. So, everything I've heard Casandra Wilson do is great, especially covering the Robert Johnson material, and U2's "Love Is Blindness", Hank Williams "I'm So Lonesome I Could Cry". Also Annie Lennox's (of the Eurythmics) entire cover album because they reinterpret the SONG and aren't slaves to the RECORD.
Robben Ford does a great job on reinterpreting things too. Check out his version of the Nina Simone song "Don't Let Me Be Misunderstood" (also covered by The Animals who did a good job too).
John