My top three are:
Black Tide - Hit The Lights, Which isn't hard seeing as I pretty much hate Metallica.
Five Finger Death Punch - Bad Company
Cancer Bats - Sabotage
Guitar/Bass = Expert
Pro drums = Expert
Vocals = Fail every song
Pro Keys = Medium
Pro Guitar / Bass = Medium
I also like 'Turn the Page' by Metallica, and 'Smooth Criminal' by Alien Ant Farm. I also think White Lion's 'Radar Love' is better than the original. That might ruffle some feathers! Great White's 'Face the Day' is much better, but nobody probably knows that is a cover song. Gun's & Roses 'Knocking on Heaven's Door' is superior. I also like Lacuna Coil's 'Losing My Religion', better than REM's. Lacuna Coil just released the album that has the song on it not to long ago, so most people don't know about it. 'Smoking in the Boy's Room' by Motley Crue is much better than the original. Orgy's 'Blue Monday' rocks better than New Order's, although I still like 'Blue Monday' from New Order. Anyway, that's all I can think of for now. Peace!
Led Zeppelin's When the Levee Breaks for sure. Much more rockin' than the 1929 Memphis Minnie original version.
Wii FC: 0852-7640-5889-3902
DLC Quickplay: http://tinyurl.com/MCSDLCQP
Last DLC Bought: Fire
Not necessarily my favorites over the original, but here are a few genre-altering covers I enjoy:
Apoptygma Berzerk - "Fade to Black" (Metallica) Hard Rock/Metal > Futurepop
Die Krupps - "Enter Sandman" (Metallica again) Hard Rock/Metal > Industrial Rock/Metal
The Flashbulb - "California Dreamin'" (The Mamas & the Papas) Psychedelic Pop-Rock > Breakcore
Laibach - "Get Back" (The Beatles) Pop-Rock > Martial Pop
Bugtoak - "Min Chuguruk" ("Highway Star" - Deep Purple) Hard Rock > Tuvan Folk/Throat Singing (yep)
_ Pure Reason Revolution
_ VNV Nation
_ More Tribe
"And onward now, and on forever, all great things to come"
Bands I'd love to see as DLC for Rock Band:
Bal Sagoth 
This. Not a huge fan of the original, but this cover is awesome. It puts the right amount of seething anger into the song, something the original was sorely lacking.
"My name is Giovanni Giorgio but everybody calls me...Giorgio.