Halo: Combat Evolved
Sure, Halo didn't invent shooters, nor did shooters become more popular overall as a result of it. Shooters are the most ubiquitous game genre for a reason. But what Halo did was define what it meant to be a shooter on a console. The speed, the physics, the mechanics, even the UI - all of these elements were so well-done by Bungie that games today still tune their games based on Halo. That's pretty impressive.