As much as I love originals of songs, I also enjoy the occassional well performed cover song. I generally enjoy when the cover is a complete 180 from the original, yet still sounds awesome. The following are a few that I would recommend.
Where did these guys ever go?
Slowing things down
Karate Kid anyone?
Some classic Don Henley action
And although all performances on shows like American Idol and The X Factor would be considered covers, I have to give this one special mention because it's the only covered version of the song that I've ever heard to be just as good as the original