Theists: What is the most compelling argument you have heard for Atheism?

[SteveII]

Regarding the probability of the initial constants, the universes initial conditions, and the laws of physics, and the objection that we don't know what the probability is:

There is a concept known as Epistemic Probability (aka inductive probability). Ian Hacking wrote 

On the one side it [the conception of probability] is statistical, concerning itself with stochastic laws or chance processes. On the other side, it is epistemological , dedicated to assessing reasonable degrees of belied in propositions quite devoid of statistical background. (Hacking, I. (1975) The Emergence of Probability: A Philosophical Study of Early Ideas about Probability. Cambridge University Press. 

A good example of this is how evolutionists say the thesis of Common Ancestry is probably true. They are not talking about statistical data. They are talking about conclusion of pieces of evidence.