4. Space Invaders (1978, Arcade)
Credit where credit's due: the game pretty much invented the concept of an alien invasion in gaming. That's how they got away with such a bland title. Anything sounds good in 1978 - particularly when the idea of shooting rows of funky aliens is groundbreaking stuff.
And that's what it was: the ever-pressing danger of destruction, as space-squid rain down from the sky with a menacing buzz-stomp. So there's no story. No sense of what you're actually defending. But when you play - just watching the menace draw closer and closer, somehow avoiding your careful shots - you'll know the true intensity of the earliest attempt by aliens to subjugate our world in a game.