QCD and only QCD, small, stable (even the dev versions), excellent range of plug-ins and some very cool skins. It's constantely evolving, it's creator really hear what users have to say.
With each version QCD becomes better (not like some other players... *cough*winamp*cough*)
I used to change player almost every month, been using QCD since Febuary and I'm not planing to change anytime soon.