Cover songs you think out did the original versions.

Personally, I reckon the cover of Desolation Row MCR made for the Watchmen film was better than the Bob Dylan original.

http://www.youtube.com/watch?v=_Ep6fe99Mps

Not sure if this counts as a reworking rather than a cover, but I also prefer the Eminem version of Dream On by Aerosmith, (Sing for the moment)

http://www.youtube.com/watch?v=jGdbt5DWRsU

And finally, Muse's version and Eels' versions of Feeling Good are both improvements on the original, ut can't find the Eels one on youtube, so enjoy some Muse.

http://www.youtube.com/watch?v=inW91qRDGwI