Splitting this by decade was a smart move!
Gotta say it's
Half-Life
. It really transformed the FPS genre and feels like a definitive turning point looking back from the DOOM style to more modern, puzzle-driven and story-heavy campaigns. The level of cohesion and scale of the adventure was nuts and the crowbar basically set the standard for what a melee weapon should be in these games.
Let the voice of love take you higher,
With this gathering power, go beyond even time!