Why does anything exist, rather than nothing at all?
Although I was raised as a strict Irish-style Roman Catholic, after many years of deep thought, I am now an atheist, humanist, rationalist and sceptic, and have come to the conclusion that this is the most important unanswered philosophical question. I do not believe there is any need to postulate a creator, because belief in a creator god leads to the obvious next questions of who or what made god, and who or what made who or what made god, and so on indefinitely.