The greatest weapon is the human mind. One can argue that the atomic bomb is a more efficient, lethal, and greater weapon, but without the human mind, the a-bomb would never have been created.
As far as inanimate objects go, I'm partial to the katana. Unlike a gun , it won't jam, and in the hands of a highly trained individual, it can make he or she virtually untouchable. (Excluding guns, poisons, and etc.)
Its not the greatest weapon in terms of efficiency, but its still the greatest to me.