Tit-for-Tat
Anatol Rapoport's four-line strategy, submitted to Robert Axelrod's tournaments, that cooperates on the first move and then repeats whatever the opponent did on the previous move. It won both tournaments decisively because it combined four properties — nice (never defects first), retaliatory (punishes defection immediately), forgiving (returns to cooperation once the opponent does), and clear (easy for an opponent to read and predict).