For me it was a golf game but it was probably ASCII only, played on a green monitor, sometime in the late 70s. I loved that game. Either that or my memory is playing tricks with me.
The original DOOM game was the first game I played with a friend where our PCs were connected together. It was a riot, especially when wearing headphones and hearing growls from behind you. I know modern tech has improved exponentially and graphics are unbelievable. But at the end of the day, it’s the experience that counts. And experiencing a multiplayer game for the first time like that, hasn’t been beaten yet, for me.