Back in 1999, Quake II was the game I played (like, literally the only one). I sunk so much time in QII and I wrote my very first code building a Mod.
I think it's fair to say, Quake II turned me from a computer user into a computer programmer. And set the course for my entire adult life and career.
From my original mod, I learned how to calculate vectors, learn an existing code base, integrate autobuilds for rapid testing.
I also dabbeled in making maps, learning 3rd art, but, quickly realized I am no artist or designer.
Sadly, my version control and backup policies at the time were non-existant and I've lost all my original code.
The next game that really got my attention in the same way was Tribes 2, the scriptings and modding you could do there was truly amazing.