I was slowly introduced to PC gaming at the time of the internet becoming a thing. I think the first "online" FPS I played was Unreal Tournament on two player LAN. Due to a combination of circumstances, this multiplayer use of the internet eluded me.
I think it might've been the first game I played online. In any case it's the one that's etched into my memory. Because by the gods had I not before experienced anything of the like. And judging by everyone else also running around like headless chickens and one after another ramming a plane into that one palm tree, no one had. Few cared about bothering to find out what you were supposed to be doing, just weeeeee! The scope, scale and chaos blew my mind. And it never really stopped.
Salute,
Battlefield 1942.