Poweramp and if you've got an SGS1 or 3 you also need Voodoo Control which will turn it into an awesome sound device.
I'm not aware voodoo sound (app) is available yet for the S3.
A workmate told me it was
PlayerPro is the best.
Neutron is the best out there, far more advanced than the others. Most importantly it has a crossfeed filter, a headphone essential. I prefer doubleTwist to PowerAMP in the more basic players, and it has a much better homescreen widget than any of the PowerAMP ones.
Another vote for poweramp, best mp3 player i have used on Android. Tried Neutron but found it too confusing and compared to poweramp, it didnt sound as good.
With a decent set of headphones/IEMs neutron is amazing (as in noticeably superior sound quality to Poweramp).
That being said poweramp has a far nicer interface and a generally more pleasant user experience.
Try doubleTwist, it looks better is is more usable than PowerAMP - which is rather fugly IMO. There were a couple of other nicer players I tried out too when I was looking for one to use.