Apollo, it has a simple Win32 interface. But it has all the features an MP3 player could need or want, with less resource hoggage. The playlist functions are cool, plus it has silence removal and crossfading built in (the new versions of Winamp might to, but winamp didn't when I used it, you need a plugin (that didn't work very well)). Oh yeah, and the AAP (Apollo Advanced Playlist) functions are sweet.
Of Winamp and Sonique, I'd definately use Winamp. I hate the Sonique interface.