Excalibur, I suppose, as long as I had the sheath that came with it. The sword literally guaranteed victory in every battle: kind of neat, if you were facing rows of tanks.
And the sheath guaranteed immunity from attack to the wearer.
But a better weapon might be the ability to enforce that, somehow, all parties to a conflict fully understood both their foes' reasons and the probable consequences of their actions. Such a mystical weapon would insure there'd be damn few conflicts.
