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

[Fake Messiah]

God works in people's lives today--changing people on the inside as well as the occurrence of miracles.

Try getting this to you head: just Polio killed 500 million people in XX. century and was eradicated in 1979 - now how did this happen? Did Virgin Mary or Jesus come from the sky and give people cure? No! It were humans, mere mortals using science, who invented the vaccine.
During that same past century medicine increased life span for 30 years. Before that whooping cough killed 8000 people a year - mostly children. Rubella caused birth defects in 20 000 children every year. Measles killed 500 000 people every year and so on and on. Who put an end to it? Was it Jesus? Again no. It was invention of vaccine.
Imagine, for instance, that in medieval times only 1 in 10 children lived to celebrate it's 10 birthday, while all knowing Jesus didn't even bother to mention penicillin that would have saved many billions of people - so if you truly want to devote your energy to a good healing force, go to science.

God is the best explanation why anything at all exists.

If that is the case then please answer me why does malaria exist? Why does leukemia exist? Why do giant earthquakes and other natural disasters exist that kill many children? I want to know, tell me if god is best explanation then answer will be easy.

God is the best explanation of objective moral values and duties.

No it doesn't. Morality and altruism comes from a simple fact that a single human cannot survive alone. From the early beginnings people realized no human is an island. Humans depend on each other's favors and also everybody has to contribute, so there really is nothing special about that.

God is the best explanation of the origin of the universe.

Big bang theory does sound complicated for people. But there is something very different about the big bang. Unlike every other "explanation" that our species has come up with for the creation of the universe, this one has scientific observations and experiments that back it up. This explanation for how the universe started is not a belief and has no need for faith.

Though I'm not sure why we have to keep repeating over and over again that many (if not most) of us Christians do not believe the OT is to be taken as word for word, literal, historical fact.

But if Genesis is just an allegory meaning that Adam and Even never existed then original sin could not be created and then what is the point of god incarnating himself as a man, Jesus, in order that he should be tortured and executed in atonement for the hereditary sin of Adam - that was only an allegorical story?
I mean how does this make sense? Maybe that's why you need to repeat your statement because you never answered this.

Not to mention that not even majority of christians on this forum takes OT as an allegory.