4. Grand Theft Auto III (2001)
Ocarina of Time may have laid the groundwork for 3D, open world games (see the Dan Houser quote on the Mario 64 entry), but GTA III brought that open world beyond the boundaries of Hyrule into a modern day city. Many critics latched on to GTA III's laissez-faire attitude towards the player's actions (a frequently-cited, oddly specific example of which was the ability to run over a prostitute to get your money back) but they missed the point. What makes GTA III so definitive is not the committing of crime but the
potential to commit crime, which sits right alongside the potential to engage in more banal, benign tasks - GTA III was as much about being a cab driver as it was obtaining sexual refunds. This concept, that there is a living, breathing world out there of which you're only experiencing a fraction, has become a touchstone of modern game design, and GTA III deserves credit for being the first title to get it just right.