Lode Runner
What made Lode Runner the must-have game to have is that it was the first game with a built in level editor.  Before Lode Runner, once one finished all of the levels in a game, it was time to move on to another game.  To make custom levels, one needed to program their own game in Basic.With Lode Runner, one could open up the level editor, easily make a custom Lode level, then playtest and share it with friends.