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

[Mister Agenda]

I disagree.  Fine tuning is an interpretation of the data.  It can't be evidence for itself.

No, fine tuning is not an interpretation. It is a fact, that the initial constants had to be in a mind-boggling narrow band of values for the universe to hold together, elements to form, galaxies to form, etc. See here for a another basic list: https://en.wikipedia.org/wiki/Fine-tuned...e#Examples

If you think this is just a fringe opinion, here is the bio of the guy who created the list. https://en.wikipedia.org/wiki/Martin_Rees

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What evidence is there of fine tuning? All we have to go in fine tuning's existence is a presupposition made by certain people that the universe exists as it is is because humanity has to exist, an unjustifiable assertion.

This has been all gone over earlier in the thread. You are wrong about how we arrive at the conclusion: 

1. The fine-tuning of the universe is due to either physical necessity, chance, or design.
2. The fine-tuning of the universe is not due to physical necessity or chance.
3. Therefore, the fine-tuning of the universe is due to design.

What is not a fact is that the universal constants could have been different. You have only arbitrarily asserted physical necessity is not the reason for the fine-tuning of the universe, not ruled it out by any rational process. I'm sure there's a Nobel waiting for the person who can do that.

Not to mention that there's no mechanism for universe generation that would not just keep spewing them out, so there's no way to rule out chance from near infinite opportunities, either.

2. fails spectacularly.

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No, they are not good arguments because the universe is not the way it is by necessity or chance. I have mentioned both and I have yet to hear back a reason why either is more probable than design.  If you think it is, make the case.

For the love of reason, you can't be this dense. You haven't given a reason why design is more probable than necessity or chance. Without that, it's 2 to one against.