Space Invaders.

[Hypercat-Z]

Also: did you know his awesome gameplay was fruit of an incident?
When his creator finished the developing of the game and tried it himself, he was frustatedly disappointed because the games was freaking slow, compared to what he inted it to be. There were too many sprites to menage for the poor CPU back then. Then he noticed that by blasting them off the remain ones were gradually, increasing their speed, because the CPU has a less heavy work to do.
By this any success of the player was rewarded with a superior challenge. So the creator of the game decided it was a good thing and the game was made as we all know.