Best argument for atheism?
This is a good question. I hope this can help clarify:
The reason I am an atheist is because I have not been provided good evidence of the theist's proposition that "God(s) exist". The burden of proof is on the theist. This would also be true if the person said they believed in UFOs, the Loch Ness monster, fairies, Big Foot, The Illuminati, etc. The burden is not on me to disprove their claims, but for the believer in the claims to prove their position. The proof of God's existence by most theists is typically faith, personal revelation, a holy book, or they point to the Universe and say "Oh, how complex...surely there was a creator", and so on. If this is not sufficient for you, you take the default position which is atheism. Therefore, the question could be reworded as "Best argument for theism?" and, if there isn't a good argument for theism, the default position is atheism.
Also, it is good to try not to mix evolution (and the Theory of Evolution), the Big Bang theory, abiogenesis, and other ideas into the mix. These are scientific explanations of natural phenomenon and are not positions for or against the proposition "There is a God(s)". The proposition that "there is a God(s)" needs to stand on its own merit (or lack thereof).
For example, the Big Bang theory is the current best scientific explanation of how the Universe came about from a high-density state then expanded. Lots of great experiments that go farther and father in time. We don't have all the answers yet and we may never have them. But the default position on these gaps of knowledge would be "We don't know" (and, maybe, "let's keep trying to find out") and not "Therefore...God(s)".
Hope this helps.