Asteroids
Rocks in space.
Of course Asteroids holds massive significance in the history of videogames, but as a game in its own right, this awesome machine demonstrated the real depths of possibility the new, and mostly frowned upon, industry had to offer.
Our opinions on what space travel might involve were decidedly one-dimensional. We had notions of populating other worlds, intergalactic war, faster than light travel and teaching new species about lurve. But what Ed Logg subtly reminded us of with his brilliant new game design is that we're human, and still will be when we invent space ships. Therefore, the majority of work done in the stellar playground is going to be menial, lonely and repetitive. Garbage collection, for instance. Between Mars and Jupiter lies an enormous belt thick with asteroids, and do you really think we, the lazy, destructive, viral human race, are going to be happy taking the long way round to the Jovian Collection?
Not likely. There'll be plenty of work for those who deforest outer space, and that's precisely the kind of realistic space exploration Atari considered in Asteroids. The apparently simple gameplay is host to such incredible and alien physics that gamers genuinely felt the claustrophobia of their tiny ship, looking out into the vastness of space and raw destructive might of inert planetoids through their small coin-op window. Equally simplistic controls hid a profundity of delicately required control as the quantum mechanics we take for granted (such as momentum and inertia) are removed from the infinitesimal equation of our insignificant lives.
Asteroids, as a game plain and simple, is especially momentous however. No matter what year this game was designed - whether it was 1979 or 1999 - it would have met with exactly the same inexhaustible success. The real legacy of Asteroids is how it proved, so early on in the life of videogames, that great games are great forever.