Richard Dawkins in the God Delusion dumbed down the ontological argument with this playground banter:
"Bet you I can prove God exists."
"Bet you can't."
"Right then, imagine the most perfect, perfect, perfect thing possible."
"Okay, now what?"
"Now, is that perfect, perfect, perfect thing real? Does it exist?"
"No, it's only in my mind."
"But if it was real it would be even more perfect, because a really really perfect thing would have to be better than a silly old imaginary thing. So I've proved that God exists."
As philosopher Immanuel Kent pointed out, the key flaw with the ontological argument is Anselm's slippery assumption that 'existence' is more 'perfect' than non-existence. Even if we concede that existence is a property, it does not seem to be the sort of property that makes something better for having it. American philosopher Norman Malcolm explained it this way:
"The doctrine that existence is a perfection is remarkably queer. It makes sense and is true to say that my future house will be a better one if it is insulated than if it is not insulated; but what could it mean to say that it will be a better house if it exists than if it does not? My future child will be a better man if he is honest than if he is not; but who would understand the saying that he will be a better man if he exists than if he does not? Or who understands the saying that if God exists He is more perfect than if he does not exist?"
To myself, being no genius philosopher, it just seems to be a form of empty, circular logic. You could easily use the same argument to try prove that the almighty creator Flying Spaghetti Monster exists.