Looking for a music player that will do a good job of loading album art. It all looks great in PlayerPro, but I tried Auxio and Aplayer but they didn't do a great job of pulling in the album art. Tracks were also incorrectly lumped together with mixed albums/artists.
I found a cool little app on F-Droid called Gramophone. It has neat little animations that make me happy, and it can color the player controls according to album art. I'm a sucker for nice UI. I was using Auxio before that. Both are good.
If you want something for ad-less streaming, RiMusic is really nice. For local audiobook playback with chapter selection, Voice is the best I've found so far.
Unfortunately, the state of Android music players is not great. Currently I have two FOSS music players installed: Metro Music Player (the F-Droid version of Retro Music Player) and mucke. mucke has a ton of really cool features that improve the shuffle experience but it's actually worse than most apps at pulling album art. Retro/Metro has beautiful UI, and has pretty good features for customization, but lacks the cool features mucke has and is less stable. Both have more than one annoying bug, but it took me a while to find music players that had this few dealbreakers.
It took me some time to get used to auxio after switching from rocket music player, but I've come to really enjoy auxio. Would recommend, unless you want smart playlists.
Having used PowerAmp since Android 1.5, I switched to Musicolet last year and it does the job just as well. It's not open source but it proudly claims that it has no network permissions, so it cannot send any data anywhere.