Herzog Zwei for the Sega Genesis. My friend and I discovered it at a gaming convention. We played for an hour and a half before the guy running the booth kicked us off it. A few months later I bought a Genesis just for that game.
Atari 2600 - The Adventures of Superman cartridge (You got to fly in 6 zones + the subway using Superman top find and defeat lex Luthor and his minions; and rebuild teh bridge he blew up at the start )