My immediate response was also Tetris, but Tetris was more of a cultural shift than a game design one. Tetris didn't help define other genres, nor is its presence felt in 3D games, or narrative-driven experiences etc. I see it like dwarf, there are multiple 'small Citizen Kanes' in gaming that did one particular thing which was revolutionary for their time, but not everything at once. Tetris was one of them for sure, but so was Mario 64. Or Doom. Or MGS. Or the first game with scrolling level design, which opened up worlds beyond the boundaries of the screen (no idea what game that actually was btw., nobody seems to make a big deal out of it).