I m using Windows Media Player 11 series it has best sound actually it include good graphics equalizer , SRS sound technique , BASS controll etc etc and as its default player , Windows takes special care of it so that it creates good sound
VLC and/or Last.fm I know, VLC is primarily for videos and last is not a "computer music player" but they're all I use to listen to music.
iTunes - I use it because I need it for my iPod, to access the iTunes Store, and I like the way it organizes my music