Kant's argument is probably the best, except not in the way he directly stated it. He did state the following via a set of statements throughout his discourse on the topic (in the first Critique), however:
To define something as necessarily existing means defining it as given in all possible intuition. But one cannot define an object into intuition. I can say the phrase "a visible five-headed unicorn" such that, "A visible five-headed unicorn is invisible," is self-contradictory. This does not mean that there are any five-headed unicorns. Vision is a class of intuition, so the point generalizes: unless we actually have an intuition of a divine nature, saying, "The always-intuited divine nature is not intuited," does not go through. We do not and cannot see or indeed have any spatiotemporally limited intuition that is adequate to the concept of the divine nature. There is no burning bush or heavenly trumpet or whatever else along those lines, that sufficiently resembles what the divine nature is supposed to be so as to certify that our intuition of such things is an intuition of God. So not even God can cause us to have an intuition of God (It might instill an incorrigible belief in Its existence, in us, but that is another matter).
Clarification: since Kant is not a pure rationalist, he does not think that substantial existence claims can be proven without intuition. Note that what he means by intuition is particular representation: our minds lead us inescapably towards a general maximum which is our concept of God, but as a generalization this is not an intuition. The point might seem obvious, then: unless we have a direct perception of God, nothing in Heaven or on Earth can show to us that God as a particular being exists.