Quake 3 Arena server
I had a surge of nostalgia recently and bought Quake 3 Arena on gog recently. The game has been a big part of my childhood, I used to go to internet cafes with friends to play q3a FFA, paying per hour, sometimes even using money stolen from my parents1.
The game has always been a pretty much perfect arcade experience - easy to learn, but hard to master, very dynamic, and honestly beautiful for how well it worked on any hardware.