Half-Life is the obvious answer, not because it used movie-level production values or deep philosophical conundrums (those would be Metal Gear Solid 2 and Deus Ex, respectively), but because it chose to integrate the storytelling entirely into the gameplay. By eschewing dialogue trees and non-interactive cutscenes, Half-Life cut right to the meat of a FPS, and gave us a peek at what video games can be capable of.
Andrew Swan, Snow Goose Productions
Half-Life introduced a new high bar in immersing the player in a world where he is the hero.
Javier Heredia, DukWorld
Half-Life was the first FPS that used the strengths of the first person view to tell a decent story. The train ride intro was brilliant.
Corey Navage, Day 1 Studios