The question was prompted by this:
My answer: The best argument for atheism is the null hypothesis that: There Is No God.
If it is assumed that there is no God unless proven otherwise - then it is difficult honestly to avoid being an atheist - because it feels like there never can be enough 'proof' for such a massive assertion.
The reason that atheism is at such high prevalence in the modern West is that this culture inculcates the assumption that there is no God as if it were common sense, necessary - and perhaps even the only sane assumption.
The reason that the majority of humans alive now, and pretty much all of them in the past, were theists is that the natural, spontaneous, inborn assumption of human beings is apparently something along the lines that god/s is/are real, important, influential in the world and in each person's life.
So atheism is one aspect of modern nihilism - the modern failure to assume, to asumme the non-validity-of (to 'problematize) what is spontaneous and natural to Man; and therefore to disbelieve in the objective reality of truth, beauty, virtue; purpose, meaning and relatedness; faith, hope and love; the need for Men to live in, and as, a family...
Our fundamental assumptions about life dictate pretty much everything else - these fundamental assumptions are termed metaphysical - and modernity teaches us that there is no such thing as metaphysics, that it is deceptive nonsense.
By thereby rendering modern metaphysical assumptions invisible, modernity renders atheism (etc) apparently unchalleangeable.
When humans accept, as they mostly do, their inbuilt knowledge and awareness of god/s - then theism is natural, and indeed undisproveable.
It is all a matter of assumptions. So the proper question moves onto considering what are the proper and valid grounds for such assumptions...
Yep! - the inbuilt, invisible prior assumption of the validity of atheism is certainly the strongest 'argument' for atheism.