Winamp 2.9x because of visuals, plugins, skins (though WA3 free form skins rock). 5.1 Sound!!! Lets see WMP9 give you a center channel.
Oh and BTW Winamp and WMP9 use the EXACT same directsound codec, so there is no way WA or WMP9 can sound different. Unless for some reason direct sound is polled diffferently from app to app.
For you people with 6 speakers you can download the 5.1 Output plugin for Winamp, its kinda buggy but you do get your center chan!! There is also an A3d output plugin too, but i can only get 2 chans to work probably because most Mp3s are encoded in 2 chans.