What do you think of this argument for God?

[Pat Mustard]

I heard an argument like this:

1. God is the greatest possible being.

2. God is a necessary being, which means that God exists in every possible world (If God exists). 

3. If God exists in one possible world, God must logically exist in every possible world. 

4. Since God is the greatest possible being, it follows that every aspect of God (being possible) exists in some possible world.

5. Therefore, God exists (in all possible worlds, including ours).

I actually just structured the premises this way myself but is the same idea as an argument I heard before. 

What do you think of it?

It is ancient first of all. Aquinas stole it from Kalam who in turn stole it from Aristotle.

With regards to this specific "way" of Aquinas, the fourth, it is sufficient to realise that it is a completely subjective argument to disprove it. First of all, who is defining "greatest" (usually formulated as most perfect), and how are they measuring it against extant objects?

Secondly just because you can imagine a "perfect" thing it does not follow that that thing must therefore exist, a subjective want for something to exist does not give existence to the thing. I can imagine the most invisible, most pink, most corned unicorn ever, yet that animal has no existence outside my imagination.