The Transcendental Argument. This argument primarily comes out of the work of Cornelius Van Til and other presuppositional apologists, and I find it to be incredibly powerful. I think it goes much deeper than a lot of the other arguments for God's existence.
When I was first exposed to presuppositional apologetics as an undergrad, I have to confess that I did not buy it. More recently, however, I've come to appreciate its coherence and value in apologetics.
The argument says that all knowledge, intelligibility, laws of logic, and so forth are possible only within a theistic worldview. On an atheistic, naturalistic worldview, all we are is matter in motion. The reason we are able to have knowledge, use language intelligibly, and even dialogue at all is because God exists and God has created us in His image. If God does not exist, then we are not that much different than soda cans that fizz. All it's doing is fizzing, and not using any intelligibility or rationality, because it is just matter in motion. Thus, when the atheist speaks against God, Cornelius Van Til compared it to the little girl sitting on her dad's lap, and slapping him. The little girl is only able to slap him, because she is supported on her dad's lap.
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