I use Winamp. I used WMP until last spring. It's really incredible how superior Winamp is. I have also used iTunes, but I don't like it very much. Winamp is more visually pleasing and the flow is very natural. It does all the simple things you didn't realize you really liked. It's also very handy with the keyboard.
One thing I hated about WMP was that it refused to play some of my files because I had taken them off of LimeWire. Winamp doesn't give a shit what my files are. It just plays sound files (and video files, too. It's great for all your .flv needs).