It doesn't really have to have full features. Just something capable of playing music continually in the background should work. I don't want to have to pay, get spied on, have something eating up a lot of space, etc.
I like simple music player by simple mobile tools, its the only one I've found that won't start playing automatically when I establish a Bluetooth connection. I used to have tasked grab the controls to avoid that but it doesn't work as a couple of os updates ago. Simple music player does exactly what I want, no more, no less, and without cute bs.