Are we to say that "God" is automatically the maximally great being? Either way, everyone should know that the ontological argument, even if it is sound, does not mean that I should pray a few times a day and condemn heretics because a maximally great being has the ability of not needing anything from humans.
Technically, anything exists, we just don't know it. God is already an recurring historical fiction character in many series. Harry Potter and Gandalf exist in the form of fictional figures. The idea exists, and the idea is as an idea as the idea about Trump or DebateArt.
If it can be made to an idea, and its existence does not automatically bar itself from coming into existence, then it exists. A being that can drive me to the town hall in a town without a hall is logically contradictory and thus does not even exist. DArt and Trump can be rationalized into existence, and we know it, thus we consider it existent.
Then, if "maximally great" is comparative, then out of all the beings, one must be the greatest. If "maximally great" is absolute, then contradictions would automatically bar it from existence, making that it is not "maximally great".