The game that started it all. Created in 1978 by Toshihiro Nishikado for Taito, Space Invaders was a phenomenon; kids were hooked and parents were horrified. I doubt anyone of my generation does not remember this game and the joy it brought. You can play in online to relive the memories.
Of course video and computer games have come light years since, only in our wildest dreams could we imagine the likes of The Sims or World of Warcraft in 1978. But none of them would have come along without this game since it proved there was a market for this kind of thing. Such a market, that there reportedly was a coin shortage in Japan when it was released in arcades there.