Marc Barrowclift wrote the most comprehensive guide to iOS music playing apps I’ve ever read:
There’s new and exciting developments every year in the realm of iOS third-party music players, and 2021 was no exception. While 2019 enjoyed an explosion of new players like Power Player and Albums that through time came to lead the space, 2020 in contrast received only a modest handful of new players and is instead remembered for the impressive growth the established player base received that year. This past year, 2021, managed to do both with a dizzying array of five new players and impressive growth across nearly all existing players.
A long, but extremely detailed and informative article. My favorite remains Marvis, due to its customizability, widget, and Last.fm integration.