The Ontological Argument - valid or debunked?

[SteveII]

This is the syllogism in Craig's words verbatim from this video:

Premise 1: It's possible that a 'maximally great being' exists. 
Premise 2: If it's possible that a maximally great being exists, then a maximally great being exists in some possible world.
Premise 3: If a maximally great being exists in some possible world, then it exists in every possible world. 
Premise 4: If a maximally great being exists in every possible world, then it exists in the actual world. 
Premise 5: Therefore a maximally great being exists in the actual world.
Premise 6: Therefore a maximally great being exists
Conclusion: Therefore God exists. 

@SteveII:
The argument assumes that a maximally great being would possess the property of necessarily existing.  If the greatness of a property is subjective, then it's not objectively true that a maximally great being would have the property of existing necessarily.  Premise 3 actually states that a maximally great being would exist necessarily.  Since it's not objectively true that maximal greatness includes necessarily existing, this premise is false, and the proof is unsound.  The only way around this objection is to show that necessarily existing is objectively great, and this you cannot do, for as explained the notion of objective greatness is incoherent.

I still have a problem with the claim that maximal greatness is subjective. We are not talking about the greatest being we can imagine. We are talking about the greatest being possible. Even if a complete picture of what that might mean is unclear, it does not matter.

I mentioned this back a few pages: The difference is between epistemic possibility and metaphysical possibility. Epistemic possibility is simply "for all we know something is possible". On the other hand, to illustrate metaphysical possibility take a math equation 24673244/8=3005567. While we might say "for all we know" this might be true, but if it is true, than it is necessarily true if it is false than it is necessarily false. If a maximally great being exists, it exists necessarily in a metaphysical sense. Therefore, God’s existence is either possible or impossible.

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One can show the absurdity of the argument by proposing an equally valid reductio in which one postulates a maximally evil being who is omnipotent, omniscient, and wholly evil as done in the following video.

Premise 1: It's possible that a 'maximally evil being' exists.
Premise 2: If it's possible that a maximally evil being exists, then a maximally evil being exists in some possible world.
Premise 3: If a maximally evil being exists in some possible world, then it exists in every possible world.
Premise 4: If a maximally evil being exists in every possible world, then it exists in the actual world.
Premise 5: Therefore a maximally evil being exists in the actual world.
Premise 6: Therefore a maximally evil being exists.
Conclusion: Therefore a maximally evil being exists.

https://www.youtube.com/watch?v=YbRSGYRQqic

All that we are doing is arbitrarily picking a set of attributes and attaching the necessarily existing tag to them.  This is no proof that a necessary being exists, whether possibly or actually.

I don't think it is even broadly logical possible that a maximal evil being exists. With omnipotence alone, that being would be required to prevent all good in all possible worlds. Since that is not the case in this possible world, I think the parody fails.