Don’t really think you can point to 1 game, but if I have to doom/doom2 would be the one. But there’s also sorts of seminal moments, I think you probably need to base it on genres.
For me, off the top of my head special mention to, street fighter 2, tekken 2, age of empires 2, sensible soccer, fifa99, proevo 2006, F197, colin macrae rally, wipeout, metal gear solid, unreal tournament, counter strike, and bringing it up to the modern era, for me battlefield 3 is a massive jump on the competition.
biggest thing for gaming I reckon was the playstation though, that was when games started to get really good and mature. Before then, grear games were few and far between and you didn’t know any better, so nostalgia kicks in(well apart from sensi and street fighter 2, they’re still great game today!)