The most fun I had, with people who give 0 thoughts about free software, were Teeworlds, Hedgewars, Xonotic, OpenTTD and OpenRCT and OpenRA, and Apple Flinger. These were genuinely fun, with relatively straightforward onboarding, and we sunk a good number of hours into them. This is not to say that they're better or worse than closed-source games, just my recommendation of what games proved to be actually fun under real life circumstances.