Windows 10 Mobile has its own native music and video players, of course, but throw in some wierd and wonderful codecs and bitrates and it can be handy to have something a bit more flexible. Enter the commercial (and seemingly always on sale!) Best Player UWP, with a truck-load of features, albeit sewn up in an interface that you'll want to (ahem, how can I put this tactfully?) 'experiment' with!

From the Store description:

All format media & IPTV player supporting EPG, DLNA, IPTV Live TV (M3U playlists), YouTube playlists, ALL VIDEO formats with any format local or online subtitles, ALL AUDIO formats and almost ALL streaming protocols, with an embedded file manager for creating playlists of Local, Network, or from OneDrive media files.

• plays almost all VIDEO formats (mkv, flv, ts, m2ts, avi, divx, mp4, mov, mpg, wmv, etc)
• plays almost all AUDIO fomats (mp3, wma, flac, ogg, wav, etc)
• plays almost all streaming protocols (hls, rtmp, rtsp etc)
• M3U playlists
• Remote playlists and auto refresh option
• IPTV Live TV with EPG support and capability to preserve data on refresh
• YouTube playlists (one click import)
• PIP mode (desktop)
• supports the most popular subtitle formats
• embedded subtitles
• Online subtitles
• Continuum support
• media casting
• full playlist support 
• drag & drop support of multiple files/folders (desktop version)
• drag & drop of subtitles (desktop version)
• creation of lists of playback (playlists) from folders
• direct playback from folders (audio or video)
• reads all the subtitles from the video file folder and automatically loads the subtitle with same name as the video file
• direct loading of subtitle from compressed files (default option is disabled)
• fully customizable subtitles(color, shadow, position, time synchronization, etc)
• supports non-unicode subtitles
• smooth playback of high definition videos
• gesture control for time position, volume control (swiping)

And as a file manager:

• can manage files easily and fast (cut, copy, move, delete) with progress display in copy function.
• multiple selection of files and folders
• fast loading of files and folders
• display of all video files, of all audio files, of all picture files in seperate folders
• automatic detection of folders that contain media files
• uncompress files (zip, rar, 7zip, tar, gzip) to the current folder or to seperate folder with progress display
• thumbnails of files
• automatic counting of files and folders that are contained in every folder
• sort by name, by type, by date
• a private folder (app's storage) can be protected with password

If you're curious about my comments about Best Player's interface then you'll get some idea from the screenshots here, though you'll have to fork out the £2 or so (sale price) if you want to invest time in getting this working well at your end.

Building a playlist, central to the way this app works, plus the detailed Settings menu...

In the built-in file manager, here adding (music) media files from folder to a playlist...

Video playback in progress, showing the full interface and playback tools...

While playback is in progress, you can start queuing up other media to follow...

Music playback doesn't exactly take advantage of the full UI - and there are minor cosmetic issues in terms of interfacing with the system media control (though it all does work properly, and in the background)...

You can buy this in the Store here. Comments welcome. I just don't know what to make of it. Usability monstrosity or blinding stroke of UI genius?