Doesn't so much spank the original as compliments it.
Alien Ant Farm's Smooth Criminal is 100 times better than by Michael Jackson.
PLUS
Mad World from Donnie Darko (Gary Jules) over the Tears for Fears original.
We've just had a scoot through far too many of the pages in this thread, and are most taken by stato's suggestions. This has been missing:
http://www.youtube.com/watch?v=eP3ge9DGsKU
This has been on, but deserves a larger audience :
http://www.youtube.com/watch?v=B6T9qp9XbRY
And, just for a laugh, why not this, especially the first 90 seconds or so, which is quite simply epic:
http://www.youtube.com/watch?v=UsFcKhxemnQ