This is actually a good question if I'm answering it entirely based on feels, not actual data.
Biggest game overall, single player plus multiplayer? Halo series. Maybe Destiny 1&2 too.
Biggest PC game? Probably Neverwinter Nights (2002). (This predated Steam, so fuck the statistics anyway.)
Biggest game I'm spending shitloads of time right now? TRAIN SIM WORLD 3.