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

[SteveII]

]Except the multiverse a) isn't science and b) does not remove the question what fine-tuned the multiverse to be able to reliably spit out trillions of universes with varying physical laws and constants?

Hello, SteveII.  If you'd be willing, I'd appreciate some clarification from you about fine-tuning.  First, is the argument for fine-tuning attributing some kind of conscious or even human-like agency to the creation of the universe? [1] Second, is it possible that the argument for fine-tuning could be inadvertently trying to make reality conform with a specific way of humanistic thinking rather than actually trying to understand how reality works? [2] Also, if fine-tuning is uncertain on a universal scale, then would it be premature and somewhat short-sighted to apply this way of thinking to trillions of galaxies? [3] 

First, fine-tuning does not equal design. Scientist realize the universe is delicately balanced (fine-tuned)--some of the constants could not be different more than 1 in 10^60. Penrose calculated that the low entropy we see in the Big Bang is 1 chance out of 10^10^123 -- and incomprehensible number. 

1. The argument make the case that design is the best answer of the three possibilities of why the universe is fine-tuned. You can infer a significant level of intelligence behind whatever tinkered with the knobs, but that probably is it for this particular argument's conclusions.
2. Whether humans had ever existed or not, the values of the constants that permit the universe to hold together, galaxies to form, the full range of elements to be formed, and space expand at a rate that does not tear it all up would still be the case. 
3. I am not aware of any debate about whether the constants apply to the whole universe--they do.