If it has zombies in it, it's a good game

and...
A game that gets you coming back to it time and time again. Doesn't have to be graphically or sonically superior to any other game - it has to be *playable*. And playable because of good programiing - good collision detection, accurate controls...and a game that gets the mind working...
Games like Sensitive on the Commodore 64. Games like Lemmings on the Amiga. Games like Tetris on the Gameboy.