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

[SteveII]

I did some reading on M-theory. First, there is not one bit of physical evidence for it. Second, it posits as many as 10^500 different possible membranes (parallel universes) that could be right next to us. With that many universes--each with a different possibility of constants, it does not address the fine-tuning issue of why ours is the way it is. Third, it is not clear that whatever created the 'cosmic landscape' (multiverse?) would not have to have been finely tuned to create universes with laws and structure (kicking the can upstairs). Fourth, since it is very much in question whether the theory can ever be tested, isn't it just philosophy and not science?

How could a multiverse create a universe without laws and structure? [1]

How would a near-infinite number of universes not address the issue of one having the values our has by chance? If you're literally asking why we're in the one that allows us to exist, you need to take a break. [2]

It's theoretical physics, which would more properly be named 'hypothetical physics'. It has to fit what we already know and the math has to work, but it ain't necessarily so. It uses the scientific toolbox, and (hypothetically) the hypotheses generated may be possible to test someday. [3]

1. It seems that you assume in your question that the multiverse is fine-tuned to spit out universes with laws and structure? On the other hand, since this isn't science anymore, why can't a universe be spawned that is just chaos and exists for a billionth of a second? 
2. So, Occam's Razor is out the window--on a grand scale. You are positing a near-infinite number of universes to explain this one. Not very parsimonious. Also, the multiverse is a metaphysical theory and will never be anything else--so any argument that has the multiverse as a component will always have that little asterisk next to it and never be, by itself, a defeater for a premise.
3. All true.