The problem of evil in my opinion is the best argument against omnipotent belief systems such as the Abrahamic religions. It seems deductively airtight, all the counter arguments are terrible, and it's the most "human" argument. It's so good that I almost think that everyone who understands it but still rejects it or doesn't care is either coping with hard shit in their life (which is understandable and tolerable), sadomasochistic, or have a bit of an evil nature themselves (which isn't tolerable).