I use the Amazon Music app for a lot of my listening, as it's obviously the only way I can listen to the stuff included with Prime.
However, for anything else (ripped CDs, stuff I've bought elsewhere) it's a bit shit - the main issue is with albums I've bought from Amazon. Both GvF albums - I have the mp3s downloaded to my phone, through the app, but I always get some missing songs, some downloaded twice, and some that are greyed out.
I used to use Google Play Music but that's gone now, replaced by YouTube Music which isn't particularly great, and is constantly pushing me to buy stuff.
Any recommendations for a one-off purchase, ad free music player?