Hi James.
There are so many possibilities that I am surprised that your question hasn't gotten a TON of responses.
I am going to assume that you have been to the Google play store to look around at the available players and probably have done a search for best android music players. So you know what the most popular players are such as Poweramp, Playerpro, Jet, Neutron, etc. Nearly all of them are available free with the option to upgrade to a more feature rich version for a small fee. I have tried all the ones I listed and I have paid the fee to get rid of ads and gain full functionality. Generally speaking they are all pretty good and your impressions are going be different from other people because it's all about how we hear things.
All that said: take your time and listen to all of the above for at least a few days each.
I have recently changed - again - to a very recently developed player called Blackplayer and it is my new all time favorite. The free version is ad free and the paid version fee is one dollar. I think it is way under priced. Add that one to the list I started and look at a few more that I didn't mention.
If I was to be pressed I would narrow it down to Poweramp Unlocked and Blackplayer EX, very heavily biased toward Blackplayer.
Hope this helps.
Recommended editorial content
With your consent, external content is loaded here.
By clicking on the button above, you agree that external content may be displayed to you. Personal data may be transmitted to third-party providers in the process. You can find more information about this in our Privacy Policy.