The ultimate weapon is one which is so devastating that the thought of ever using it is enough to end most conflicts.
The atomic bomb is THE ultimate weapon of modern warfare.
The greatest military hardware is one of these. Or more accurately, a swarm of these.
Ni it is clearly not the ultimate as it depends on you getting it some where, if someone develops a near fool proof methods of stopping icbm, and other forms of delivery, then it immediately becomes useless, till something new is made.