Poweramp is yet another Android-only music player app that I will share some thoughts on based on my experience. It has a huge download base, and had been actively developed for a very long time. It is currently in it's third major revision, and in my opinion is one of the best looking UI's available. And for those with premium devices that support hi-res playback with built-in or eternal DAC's, this is a serious contender as one of the best.
The REALLY Good:
Poweramp is pretty nice looking with-out-of-the-box default settings. There are plenty of options to allow you to tailor the two views to your preference (a light and dark mode, each with selectable settings). If these don't suit your taste, there are several 3rd party skin apps out there that range from a minimalistic design scheme to one that can change virtually every displayed aspect of the app. So plenty of options to get it "just right" for your application.
The library browser is also pretty top-notch. With folder-based and tag-based options, most users will be happy with how this is managed for them. And drilling down is quick and the transitions are smooth and seamless. You can choose from many list views for each heading as well, with different text sizes and thumbnails available. I have a library of 25k files on a dedicated µSD card, and fast scrolling through them doesn't choke Poweramp on my older Android DAP (version 8.1).
The Poweramp library is also independent of the Android built version. It has support for multiple genres, artists, etc., and you can even define one or more separators based on whatever you have used in maintaining your metadata. This is a pretty significant feature that makes a big difference when it comes to managing my library on the go. Only GoneMAD seems to have integrated this idea, but lacks the option to choose the separator.
For sound tuning, there are many extremely powerful options with several DSP effects (Tempo, Stereo Expand, Mono, and a full compliment of Reverb options). More importantly, you can choose to use a graphic EQ with up to 32 bands, or go for the ultimate control with a parametric EQ that can also have a ridiculous 32 bands (with Low Pass/High Pass, Low Shelf/High Shelf, Band Pass, and Peaking Band filters, and user selectable Gain, Frequency, and "Q" adjustments). I can't think of too many other apps that offer this much tuning combined with a very user-friendly UI to control them. A huge plus in my opinion.
What Can Be Improved:
One of the few things that Poweramp does not currently customise is the metadata displayed in both the browser and Now Playing screens. All have a main line that shows TITLE and a smaller secondary line that displays ARTIST - ALBUM (or in some optional cases ALBUMARTIST - ALBUM). It would be nice to have some flexibility here, changing some of these tag fields to any of the other key tags you may prefer to see.
Sonically, Poweramp sounds fantastic thanks to the tuning features, and the proprietary audio engine that has been developed completely outside of the standard Android engine. It is almost on par with Neutron, with just one exception. It currently does not have an option to "follow" the source frequency. For most without hi-res capable devices, or if your library mainly consists of CD rips (choose 44.1kHz) or downloads (choose 44.1kHz or 48kHz to match the majority of your library), this may not even be an issue. [Note: This will be device and headphone/DAC dependent, most Android devices without hi-res DACs are internally capped at 48kHz]
A couple of other missing features that are notable include the lack of a "Smart" playlist function, and no support for the sorting tags for Artist, AlbumArtist, Album, Composer, or Title. The tag editor is pretty limited as well, really it is designed to simply make quick fixes on the fly. And while Poweramp does have basic lyrics, this could be improved.
Poweramp is a well-built player for modern Android devices. It checks almost all of the boxes for sound quality, and the UI is robust and quick. The few minor critiques will likely be addressed in future development updates. If you are on an Android device and haven't taken Poweramp for a spin, I highly recommend it.