This question would apply only to those who were once religious, as all humans are born as atheists but often converted young. I’ve written up my personal answer to this question for a book being produced by Minnesota Atheists, and I’ll keep you posted on that. In the mean time, Greta Christina is running an internet poll in which she asks this question.
This is not a multiple choice poll. It is more of an essay quesiton poll.
This is what I think is funny: What if PZ Myers Pharyngulats this poll!!! Hahaha. Anyway, go and stuff the ballot box with your considered opinion: What Convinced You? A Survey for Non-Believers