I was a big fan of WMP 9 and used it pretty much exclusively. Then 10 came out and I don't care for the interface at all. I think 9 looked much better. I like the library management, but am so anal that I keep my library organized meticulously as it is.
I have now discovered the joy that is foobar2000. Low resource use, great sound quality, and customizable out the wazoo. No radio, so I still have iTunes, WMP 10 and Winamp for that, but to play my files foobar is king.