nandohyphen1 VLC Android can play FLAC - and basically any other file format.
This is probably the best answer; it really can play anything. That said, if you're a real music aficionado (especially classical, which is where FLAC really predominates), you'll probably need gapless playback, which it doesn't support. foobar2000 is the best for that.
Now that phones have enormous storage, I just use syncthing to get my whole flac library on the phone and use f2k for playback. Works brilliantly. It's available from Play Store or as a direct apk download from https://www.foobar2000.org/
Been around forever (I started using it in college in 2001) and hasn't changed much. Just works.