Half-Life 2 (2004)
Valve's futuristic first-person shooter deserves a place in the videogame pantheon just for the introduction of the Gravity Gun. That single innovation cemented a change that had been brewing for a long time. The ''environment'' wasn't just something you walked through while killing people; post-Gravity Gun, it was an organic part of the experience, and it could be the deadliest weapon of all. But Half-Life 2 also set a new benchmark for in-game storytelling, eschewing cinematics in an immersive storyline. (It's remarkable how few games took the basic lessons of HL2 to heart...and it's depressing when an otherwise-stellar modern game like Assassin's Creed 3 ends with what amounts to a neverending poorly animated cartoon.) Best of all, because Valve encouraged players to create their own modifications, Half-Life 2 became a veritable laboratory for user experimentation.