This is going to be a bit random, but Arma 3. The base game itself isn't that crazy, and with it's realistic military focus it's not for everyone.
That said though it's completely moddable. And some of the mods out there basically turn it into a whole new game.
Currently one of the most popular mods is Exile. Another is Battle Royale. Exile is definitely a game that requires you to have some time to play, Battle Royale on the other hand is good for a quick 20-30 minute game.
Path of Exile is a great Action RPG that's free as well.