I still remember playing multiplayer deathmatch (more than 1v1) over modem, with ~200 ping, and still feeling how crazy responsive it was. That was as transformative of an experience much like seeing SuperMario 64 for the first time.

Quake multiplayer was where I had to abandoned playing "closed style" on a keyboard and instead adopt the WASD mouse and Keyboard style.

Also remember when GameSpy was QSpy, and PlanetQuake was just a fan site?

And the fun of playing Threewave CTF with the grappling hook.