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Rolf Andreassen just posted on DSL a new rebuttal of the ontological argument:
It occurred to me that the ontological argument asserts, as an axiom, that "god is the most perfect being", and then goes on to argue about existence being more perfect than nonexistence and so on. But this is begging the question: The point in dispute is precisely whether the god exists or not, and thus to say that it "is" anything is assuming the conclusion. The formulation should be "the god, if it exists, must have such-and-such properties..." and then, of course, the whole ontological argument boils down to showing that the god, if it exists, must... exist. Which is a bit less profound.
This seems different from the standard rebuttal, impressively short, and sufficient. If merely replacing the premise "god is X" with "if god exists then it's X" is enough to break the ontological argument, it's clear the argument was a swindle. Good work.