Tit for tat

Cooperate first, then copy what the other player did last time.

Simple, and famously successful in repeated Prisoner’s Dilemma tournaments: it is nice (never betrays first), retaliates, forgives, and is easy for others to understand. A slightly forgiving version does even better when mistakes happen.

Tit for tat never beats the player it faces: at best it ties. It does best overall because it gets others to cooperate. It is not unbeatable either; when moves can be misread, other simple rules can do better.