The Ultimate Weapon is a cipher. An indeterminate unknown amid a field of known knowns that simultaneously embodies and fails to embody our hopes and dreams / fears and anxieties until proven to/not-to.
It requires 2 AA batteries.
Generation One cartoon continuity
Galvatron hoped that the Ultimate Weapon's unknown nature would cause Rodimus to project all his anxieties and insecurities onto it. Instead the Ultimate Weapon helped Rodimus to conquer his doubts by externalizing them. Since logic dictated that if Galvatron had an Ultimate Weapon he would have used it long ago... the weapon had to be a fake. He had been coddling his own insecurities.
Rodimus observed that the real Ultimate Weapon was the mind.
Galvatron, pissed off, broke his toy and ran away.