Valve's first game and still their magnum opus. Setting aside the narrative aspects that pretty much every shooter afterwards took to heart, I think what sets Half-Life apart even today is how it uses a relatively basic set of mechanics in so many ways. Level design has secretly always been Valve's ultimate ability, and Half-Life just shows it. There's a specific freneticness to the combat and movement that constantly asks you to push forward. It somehow does spectacle better too than most modern games by how it really just nails the player's mindset during. There is not a wasted moment in Half-Life, and that's what its legacy truly should be. Xen doesn't count.