I've basically tried all of them... the best one IMHO is GSPlayer for the folloiwiong reason:
1) Best audio quality, due to more filters (things like EQ, bassboost, surround, and others)
2) Partial Screen--I have this assigned to a buttion, and then when open, that button hides the app. That way, I can be using any application, click that button, check out the playter, enter some commands, and then hide it again, instantaneously
3) NO background play hiccups
4) Skin support, good for customization freaks like me
5) Ogg Support, most compressed audio file siize for quality tradeoff out there.
5a) The filters all work for the ogg files
6) It's free
http://hp.vector.co.jp/authors/VA032810/gsp.htm