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 the way it brought them together and the degree to which it polished them to arrive at something fun as hell was nothing less than revolutionary at the time. It was so groundbreaking 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.