Just to throw my 2 cents into the ring, I've had the opportunity to use Windows Media Player, ITunes, and Zune to manage my music. Of the three, I think WMP is by far the best in terms of the users ability to edit info (track, artist, etc.). Zune is pretty close to WMP in this regard, but comes with an added bonus. When I download music from Amazon and direct it to WMP, it automatically updates the library in WMP and Zune. When I download music from iTunes, it automatically updates the library in iTunes and Zune. So Zune is nice in that I never have to refresh the library, and it plays both formats fine. iTunes feels very limiting and somewhat visually archaic, but it's the best place to put my music if I want it to integrate with most of my devices/services.
I guess in summation, Zune may be the best player out there. But its critical limitation of only working with Zune devices means that WMP becomes the best by default. iTunes is good enough, but it's hard to compliment it. Nevertheless, guess what my default player is?