DOOM.
Seriously, that game changed my life. I had played maybe one or two other shooters (Hello, Goldeneye!) by the time I played my first DOOM game in DOOM 64 way back in 1997. From there, it was a glorious dive into one of the most influential games of all time -- I recall being afraid of the bloody faces the HUD from the original DOOM games had, but I had no idea that years later, I would be playing it again on PC with a mod that turns everything into bloody chunks. Having played so many games throughout my life, I like that feeling of being directly in the game, where my actions would actually have effects not just on my game avatar, but on
me -- Sure, watching events happen to Link or Leon Kennedy was exciting, but in first-person? It becomes easy to forget the "player avatar" part and get immersed in the game events themselves. Also doesn't help that shooters are still popular today -- Fortnite may be the biggest game on the planet right now, but it wouldn't be anything without DOOM to pave the way first.
I have a few other games I could say 'changed' my life in ways. Super Mario World was the very first game I beat as a child, and it let me know of the finite nature of the universe, heh. Super Mario RPG got me into, um, RPG's and their deep gameplay mechanics/story/worlds. Grand Theft Auto III, Vice City, and San Andreas each got me through rough patches in my life. Fallout 3 introduced me to the idea that, yes, modern games could still be good, prompting me to continue gaming to this day. Hell, the Mass Effect series has all of the above-mentioned qualities (seriously, the multiplayer components for 3 and Andromeda contain a gameplay loop so satisfying, I'm still playing them -- now if only they'd do something about those microtransactions, heh...), where I would wager money that an inspiration for its combat mechanics were inspired by DOOM -- visually distinct enemies and enemy types to allow for quick target identification and strategizing, a wildly varied arsenal that isn't held back by realistic conventions, emphasis on mobility, and a specific atmosphere that fits the universe. Sounds like classic DOOM to me!