I'm a fan of the Limp Bizkit cover of Sanitarium by Metallica. I don't know why, I just really, really like it - more than the original, even.
I can't give that my seal of approval =/. But then I'm a huge Metallica fan...have been since I was in like 3rd grade. Cliff Burton could single handedly destroy Limp Bizkit. I'm a huge Primus fan too, and I still much prefer Metallica's version to this:
While we are on the subject of Les Claypool, the Fearless Flying Frog Brigade does a hauntingly good cover of Pigs (Three Different Ones):
I shouldn't have to tell you all that it's a Pink Floyd cover.