Fallout 3

Bethesda's 2008 revival moved the series from isometric turn-based tactics to a first-person open world, and the argument about whether that was a gain has never really finished. You leave Vault 101 into the ruins of Washington DC, rendered as a grey-green expanse of collapsed metro tunnels, super mutants and settlements built out of scrap, and the main quest about finding your father is mostly a pretext for wandering. Its signature system is VATS, which pauses combat to spend action points on targeted shots, splitting the difference between shooter and RPG and providing the slow-motion decapitations the series became known for. The writing is at its best in self-contained places rather than the central plot: Megaton and its undetonated bomb, the simulation in Tranquility Lane, Andale. Where it is weakest is in choice, offering fewer genuinely different resolutions than the games either side of it. Its lasting importance is structural - this is the template Bethesda has been iterating on ever since, through Skyrim and Starfield.