Quake 3 was my first but Ultima Online is the one that made me fall in love.
OG Fallout.
In early 2000s, I got hooked into an online MMORPG called Tibia with some friends. At it’s peak it had something like 70k players online. It was cutthroat, if you died you’d lose hours and hours of progress. I was hooked at one point doing 14h days. That experience has been impossible to recreate in adulthood.
For me it would have definitely been an MS-DOS game. Exactly which one is harder to remember since I was a kid, but I do recall playing a lot of an educational game called Operation Neptune.
The first game I ever played was Mario on the NES, but the ones that really got me into gaming were Duke Nukem 3D and Quake on PC. It’s been 27 years and I still enjoy them.