There are a couple of good music players on Google Play, many of them have a cool UI too. But the best, I think, is Timber.
Timber is an completely open source music player app, which is currently still in beta (but very stable). In addition to this you'll get a great support because development seems to be very active.
In my opinion the best thing is that you can choose the player layout. And you can also customize the UI colors.
If you have an Android Wear watch or an Android Auto compatible car, Timber has support for them too.
The app is just 6.3 MB big.
By the way: If you know some other good apps, just leave a comment in the comment field! Thanks!