Battlefield 1942. Vehicle combat, area-control mechanics,"realistic" shooter gameplay (before that term became an obscene word), and class-based team mechanics had all been invented before, but it was the first game that brought them all together in such a polished and fun-still-hard-to-equal-today package that (for better or worse) it basically spawned the "AAA WW2 game" genre that then lasted for decades.

Then, the sequels were so consistently mediocre that the original was more or less erased from history.