there are a nunmber of weapons that, during their era, were able to dominate the battle field for generations, sometimes for 300-400 years. some of these were revolutionary, soem were direct reposnses to other weapons.

examples -
the chariot
the hoplites heavy spear and armor
the roman pilum and heavy choping sword
the central asian composite recurve bow
the English longbow
ships with triangular sails and cannon
the rifled musket with ring bayonet and percussion cap
the chain fed machine gun
the tank
the AK-47

if I were to choose one, it would probrably be the AK-47. I am not sure that any weapon has enabled as much strife or has given untrained, physically weak people as much power as the AK-47.