You know, I always found particularly comical the argument that God is the greatest being conceivable, and a being that exists is greater than one that doesn't, ergo God exists. No offense to all of you, but I prefer fictional beings to real ones. Thus I singlehandedly prove that God cannot exist, as if he did exist he would not be the greatest being conceivable.
I'd actually like to see somebody debunk my logic. I think Anselm was right that the Judeo-Christian concept of an omnipotent, omniscient god does imply that such a god would be the greatest being conceivable, and it's an empirically observed fact that I feel a fictional being is greater than a real one, so you would have to find the flaw within the reasoning itself. However, I can't identify any hole in the core logic other than that one cannot sensibly make conclusions based on the properties of potentially nonexistent objects; however, in this case, either God exists and the argument holds to prove he doesn't, or he doesn't exist and the argument is unnecessary. I know there must be some other flaw, since disproving God is just as absurd as proving God, but I can't find what it is.