Previously in the comments someone said that there's been no Citizen Kane of games. Something I've heard said before, except the last time it was Birth of a Nation. The argument is that games are just sideshow entertainment and we're waiting for the one truly groundbreaking one that will be Art or Be Truly Inventive or Have Lasting Importance.
But we've had a bunch that qualify! Braid and Grand Theft Auto 3 are works of art. (Surprised that I put GTA in there? The way freedom permeates the whole thing, from the name of the city to your actions to the gameplay, and the exploration of the fuzzy area between freedom and criminality, makes it a work of art in my book.) Games that are truly inventive are too numerous to list-- when they come along they spawn genres--Dune 2, Doom, GTA (again), The Sims, Guitar Hero. Civilization has lasting importance - it gets my vote for our Birth of a Nation - think I learned more about ancient history because of Civ than any history class I ever took. (Side note: Civ Revolution is the best thing that's happened to my DS, and I'm pleased to announce that I just beat it on Deity last night for the first time.)
Ok, so those are mine. What are your Citizen Kane / Birth of a Nation of games - and why?