Source engine for Quake combined with the LibreQuake assets.
While FTEQW does have a launcher, if requires that you have Quake assets available locally. It won't launch the game without it, which means you either need the original game data of open assets such as LibreQuake.
However, if you connect to the game server directly, FTEQW will download the required assets automatically from the server.
From the terminal, type (on a single line):
fteqw-sdl2 +connect play.onfoss.org +set com_gamename "" +set vid_fullscreen 2 +name "YourName"
Of course, substitute "YourName" with the in-game name you want.
It will complain about missing files, but you can ignore that. Just press <Esc> and you are on your way!