AD&D
I haven't played AD&D in a long time, which is trully unfortionate. AD&D is the forefather of modern Role Playing Games. I used to play regularly, even started an AD&D club in highschool. Since I have come back from Israel (almost 4 whole years now) I have completed one, and been in a total of two campaigns.
AD&D, and its progeny, are some of the greatest ways to excersize the imagination, a trully under-used portion of most of our minds.