Best cover song ever done was Jimi Hendrix covering All Along the Watchtower.
Most attempts to cover Hendrix on the other hand fail badly, because almost nobody can seamlessly produce the wide range of sounds from the guitar that he could. Internet guitar nerds think Hendrix is no big deal because they can hit all the notes on the paper. Problem is, they all sound terrible doing it, because they're just working in such a narrow range of sounds compared to him, so you hit all the notes and it sounds unbelievably flat by comparison.
