I have a special place in my heart for Winamp's classic UI. In my opinion, it was the most intuitive interface for a music player. it had a playlist, equalizer, plugins, all of which were condensable to a fraction of it's full size. Of course, now there are a number of similar players that have the same capabilities, but shit, Winamp 2.x was where it started. Even today, it blows todays iTunes interface way out of the water.