Quake III Arena. It came out in the 20th century, which sounds crazy to me. It was about 10 years ago I last played it or its remakes.
I got into Q3 a few years late, but there was still a fairly healthy local community for it back then. Quake 4 came out and then Quake Live, which I played for a while, but the local community was missing by then.
Sure I could still play it, but I’m guessing there aren’t even enough people to fill a 5v5 capture the flag match anymore, Q3 or QL… a shame. But at least I have good memories of it.