1. 'The Elder Scrolls IV: Oblivion'
The Elder Scrolls series went small-scale with 2002's Morrowind, crafting an exquisite fantasy RPG on a single island. By contrast, Oblivion seemed designed with the commandment to go big or go home. Its setting was the entire nation of Cyrodiil, every forest, city, and river. The memorable moment you leave the sewers and get your first, blinded-by-the-sun glimpse of that world would be copied by other open-world games like Fallout 3. Its fully voiced NPCs would also become standard, though thankfully the acting would improve. Oblivion wasn't the first open-world RPG, but it changed how they looked and sounded forever.