Final answer:
The most convincing objection to the Ontological Argument is that it could be used to prove the existence of a greatest conceivable evil being instead of a greatest conceivable being or god.
Step-by-step explanation:
The most convincing objection to the Ontological Argument is the idea that it could be used to prove the existence of a greatest conceivable evil being, rather than a greatest conceivable being or god.
According to this objection, if we can use the same arguments to prove the existence of an all-powerful god, we can also use them to prove the existence of an all-powerful devil.
Because there cannot be two all-powerful beings, this objection suggests that the Ontological Argument fails to establish the existence of a god.