Half-life.
Because I didn’t even play games until I installed that for my brother and sat transfixed at the screen ‘to make sure it was working’ for the train ride into Black Mesa while my brother said ‘can I have a go?’ for the next while.
At that time, it was an order of magnitude greater than anything else I’d ever seen, it had real atmosphere, amazing action and epic set pieces with new AI but it also had subtlety and pacing, all the stuff we expect now, that games just didn’t have back then, and it was all new, original. I still clearly remember the first time I launched the rocket, the music, ‘We’ve got hostiles’, Dr. Kleiner and co., my crowbar (sorry doc). I remember crawling through a vent when a headcrab jumped out and bit me, I jumped in my seat, it was the first had gotten to that level of immersion for me. It felt real. Even HL2, technically better in ever regard, didn’t feel as original to my senses as the memories of Half-life. Best game ever.