I'm sorry but Sheryl Crow had NO BUSSINESS what so ever doing a cover of Guns "N" Roses' Sweet Child of Mine. Don't get me wrong, I think Crow a talent songwriter and singer so it was sad and overall disappointing the way she just TOTALLY BUTCHERED that song.
Also I remember when I first got my copy of Disney's Jungle Book "Special Edition" DVD. I was checking out the "special features" and could not believe how AWFUL the Jonas Brothers made King Louie's song-"I Wanna Be Like You." At the time, I really didn't know much about the Jonas Brothers, so I check it out and I could NOT make it all the way though. They also did an AWFUL just AWFUL verison of Ursula's (From the Little Mermaid) "Poor Unfortunate Souls." Those are GREAT Disney songs and I understand the Jonas Brothers are the "hott" band from Disney right now, but please keep them away from the classics,
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Also Celine Dion did a cover of AC/DC's "You Shock Me All Night Long" and it sucked.
Mariah Carey did Def Leppard's "Bringing on the Heartbreak" and that was tough. I'm sure Def Leppard didn't have a problem since it became a huge hit for Carey and therefore made Leppard some more money, but still, it was BAD.
Never heard any of Miley Cyrus' cover songs and I'm pretty sure I don't want to.
As for "best covers" hmmm, that's a one to think about.
Since people have already mention Muppets into the fold, can't forget about their AWESOME verison of Queen's Bohemain Rhapsody.
Ozzy Osburne did a pretty neat cover of Bee Gee's "Stayin Alive."
I enjoyed Mariyn Mason's cover of "Tainted Love" originally by Soft Cell much more than his cover of "Sweet Dreams (are made of these)" oringinally by Eurythmics. But his best cover would probably have to be Danny Elfman's "This is Halloween" from the Nightmare Before Christmas.
If you're a fan of "punk" music, check out the band "Me First and the Gimme Gimmes." They do some pretty cool covers, one of which was Rainbow Connection which is heard at the end of "Kermit's Swamp Years."
Speaking of Rainbow Connection, this song was covered very VERY well by Willie Nelson and Sarah Mclachian both.
Ray Charles also did a great cover of "Bein Green."
Steven Taylor of Aerosmith did a really cool and rocking cover of Oscar the Grouch's "I love Trash."