THE BEST cover tune EVER - Led Zeppelin - "Babe I'm Gonna Leave You".
I agree with Jon (It's my second favorite) - FNM's "War Pigs".
I also like:
Seether - "Careless Whisper"
Counting Crow - "Big Yellow Taxi"
CCR - "I Heard it Through the Grapevine"
No Doubt - "It's My Life"
Tesla - "Signs" and "Mother's Little Helper"
Spiderbait - "Black Betty"
Johnny Cash - "Hurt" (friggin' awesome)
AND
David Cook's cover of Chris Cornell's cover of MJ's Billy Jean *whew*
38 Special - Rockin Into the Night
Velvet Revolver - Cant Get It Out of My Head
Ace Frehley - Fox on the Run
Atlanta Rhythm Section - Spooky
Molly Hatchet - Dreams I'll Never See
Santana - She's Not There