Mpv

mpv is a fork of mplayer2 which in turn was a fork of MPlayer. Major improvements include optional built-in mouse-controlled OSC (on-screen controller) in addition to classical non-interactive MPlayer style OSD (on-screen display), optional color management support and optional frame interpolation to smooth out movement of typical low frame-rate content and more importantly avoid judder when time between display refreshes and frame changes is not constant (which almost never is). Both color management and frame interpolation require use of OpenGL video output driver. Smaller changes involve highly configurable video scalers and support for various hardware decoding and presentation solutions and specialized output hardware such as RaspberryPi. These are changes since forking and some of the features may also be found in MPlayer nowadays. It has similar but not the same configuration and command line format as MPlayer as such it may or may not act as a drop in replacement for MPlayer depending on how it's called but the two applications can co-exist on the same system as their files do not clash.

Emerge
Install :

For YouTube and select other streaming website support also install (soft runtime dependency):

Configuration
Historically per-user configuration was placed in mimicking MPlayer but nowadays the recommendation location is. The overall format has is the usual INI inspired MPlayer config syntax with optional profiles that when correctly named can be auto-loaded depending on situation but with certain limitations stemming from the architecture of the application. Because options change over time and there is too many of them it's always best to read mpv manpage (man mpv) yourself.

Example user mpv.conf
This is just an example, feel free to tweak it or use the defaults and remember to consult man mpv if you have any questions.

Usage
A bit ugly due to mix of different CLI formats but a good example of a more advanced mpv use case:

mpv [-profile headphones] [--loop=inf] [ ... ]