Best cover EVER:
Jimi Hendrix - All Along the Watchtower (originally by Bob Dylan)

Hon. mentions:
Metallica - Whiskey in the Jar (traditional)
Guns 'n Roses - Knockin' on Heaven's Door (Bob Dylan)
Alien Ant Farm - Smooth Criminal (Michael Jackson)
Muse - House of the Rising Sun (The Animals)
Reel Big Fish - Take on Me (A-Ha)

Icky-bad covers:
Madonna - American Pie (Don McLean)
Limp Bizkit - Behind Blue Eyes (The Who)
Guns 'n Roses - Sympathy for the Devil (Rolling Stones)
Will Young - Light My Fire (The Doors)