Halo: Combat Evolved.
It's one of those games where it's hard to overstate how much of a breakthrough moment it was if you weren't there.
The scale, the graphics, the music, the AI, it was on a whole other level from any other shooter yet made to that point. It revolutionized the genre to the point where if you pick almost any current gen FPS game more than two decades later, it will owe much of its DNA to Halo. What Mario did for first 2D then 3D games, Halo did for first person shooters.
It practically single handedly carved a place for Xbox in the console space, and set the tone for much of the 2000s as a decade. But most importantly, it was just a blast to play.