Grand Theft Auto III
Grand Theft Auto had played around with mature themes and open world environments on consoles before, setting them aside from most non-PC games, but when Grand Theft Auto III hit, things changed. Everyone had a concrete example that they could point to in order to justify rich narratives, open worlds, and games made for adults. The industry was keen to follow.
What I loved was that the game never forced you to do anything. Thanks to the cheat codes that you could find right here on this site, you didn’t have to work for the zaniness. You could drive around, listening to tunes in a tank. Or hop in a helicopter. Of course, engaging with the story is also a possibility.