Most weapons have just been developed to get one up over the enemy in a given situation. They may have shaped a battle or two, but rarely do they change much more.
The one weapon that has changed the world more than any other is of course the nuke. It has changed the world not only by forcing changes in military doctrine, but also politically, ideologically, socially and more. No other weapon can come close to the effect that nuclear weapons have had on the human race.
If talking about more conventional weapons, however, then something along the lines of the first gunpowder rifles, or the machine gun, would be at the top. To give individual people such a destructive force in the time of cavalry and swords was monumental. However I don't think it could be narrowed down to an individual weapon.
The one weapon that has changed the world more than any other is of course the nuke. It has changed the world not only by forcing changes in military doctrine, but also politically, ideologically, socially and more. No other weapon can come close to the effect that nuclear weapons have had on the human race.
If talking about more conventional weapons, however, then something along the lines of the first gunpowder rifles, or the machine gun, would be at the top. To give individual people such a destructive force in the time of cavalry and swords was monumental. However I don't think it could be narrowed down to an individual weapon.