UT2004 (with a tiny splash of UT99). This was at a time where "the internet" became mainstream enough to be affordable enough to be on it full time and speeds started being "genuinely less bad". My friends didn't play, but I found friends online. Was hesitant to join a "clan", but started discussing on forums...which I still do, last time I checked.

As I hung out on the same servers and forums, we bonded over our love of the video game. I reviewed maps, dabbled into making a few and played SO. MANY. CUSTOM. GAMETYPES. Hung out with a loose bunch of US people I considered friends (meaning: around 3AM in my timezone). Hung out at "local" (meaning: UK-based) onslaught maps.
Joined a clan, on the premisse it wouldn't be serious competition. Liked it. Helped a clanmate win a mapping contest. Won a small writing contest (on the forum...about UT2004). Went to a few specific LAN-parties, which also were among my first abroad travels as an adult (and first, the first time, considering I hadn't seen anyone face to face back then).
Through all that, it also dragged me through the sh** that is being unemployed. I dunno...I always presumed that once you had a grade and applied, the rest would come easy. It didn't. On hindsight, if I had gotten a job sooner, it wouldn't have been as life-altering. But it wouldn't be as bittersweet either (fuck...for that first LAN, I attempted to buy some UT merchandise from a shady shop through the pay credentials of a clan mate. I paid him back, but that merch never arrived. And while pocket money now, that really tore into my disposable income back then).