What do you think of this argument for God?

[Angrboda]

Yeah, I've heard this one before. I don't accept the first two premises to be true. The third premise is a guess that God exists in any possible world in the first place, so I can't accept that to be true either. The rest crumbles with that flawed foundation.

The first premise is true if the theist defines God that way (after all, you can't refute a definition unless it is logically inconsistent). The second just means that since God is necessary (must exist), the existence of God implies existing in every possible world (logically impossible to not exist).

The first premise is incoherent if by greatest one means objectively greatest. Greatness is always the function of a value system. Something is great according to some set of criteria. There are no properties which are great in and of themselves. (And if by greatest one means subjectively, then the whole argument collapses.)