I joined and ran many clans growing up but my first was in Rainbow Six: Rouge Spear. I spent many of hours sniping on bunkers and swearing at each other over Teamspeak. One guy I met took me under his wing and showed me all about each level. The mod scene was pretty good too, granted you had to manually install the mod locally or the server wouldn't let you in.

My most memorable game was America's Army. The challenge of the gameplay really drew me and my RL friends in and we ended up making a clan. We liked it so much we rented a legit server so we could get honor on our own turf. Between us and the players coming in it was some of my best time on a shooter.

Just for sheer amount of hours, Counter Strike 1.6 reigns supreme. I remember when I first saw my America's Army friends playing it and was in awe. The movement was janky compared to America's Army, but had way faster gameplay and more weapons. At first I had trouble getting the hang of it and it took a while but eventually I could somewhat keep up with them.

Needless to say, I got zero sleep as a kid.