Halo 2 (2004)
There's a solid argument for any of the first three Halo games to be on a list like this, but Halo 2 arguably moved the needle for video games as a whole more than any other game in the series. It singlehandedly set expectations for what all subsequent online shooters needed to strive for, and it laid the foundation for Xbox Live as we know it today. Significantly, Halo 2 introduced the modern interpretation of "matchmaking" in online games. Rather than individuals setting up games and inviting people in, Halo 2 introduced a playlist system that automated the process and let players move through a steady flow of available games that were also filtered by a skill ranking system. Mechanisms that we take for granted in everything from the latest Halo games to Gears of War and Overwatch today – like party systems, clan support and friends lists – were pioneered on the original Xbox in 2004.