Android's default video capabilities leave a lot to be desired, so the Play Store has a small but thriving industry of third-party video players. MX Player has been one of the most dependable among them, and the latest update fixes a few bugs on Android 5.0+ devices and adds a few new features. The most notable is probably the new ability to upload and download subtitle files from the web. That's a big deal if you often watch videos in a language you can't speak - anime fans, ahem, accessing unavailable shows come to mind.
The default function uses the language from your phone to search opensubtitles.org, either with the name of the video file or with a separate manual text search. For those of you with constantly-connected keyboards, the "up" and "down" keys can now be assigned to specific actions on the Now Playing screen, and the player UI's auto-hide feature can be disabled (but still manually hidden). If you prefer, the next and back buttons can be reset to perform fast forward and rewind functions, in case you prefer not to scan through the progress bar, I suppose.
Finally, the latest version of MX Player fixes a broken HW+ decoder that was giving trouble to some users on Android 5.1. MX Player is a free, ad-supported download, but if you'd like to support the developer there's an ad-free version available for six bucks.