The Ontological Argument - valid or debunked?

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Which it is. At best, God can be super powerful to the point that no other entity would ever exceed it in power, but not literally omnipotent.

You are not pointing out why a maximally great being is illogical.

Existence cannot be predicated...it's begging the question.

Ex. Possible purple apples exist.  The very postulation concerning this apple in question presupposes the apple's existence. In effect, making the claim that the existing purple apple...exists.    (or in this case an existing god, exists!   )

On top of that add the vagaries inherent to such abstract terms such as "all-powerful" ...and this dog don't hunt.