I caught this some time ago on Jonathan Miller's excellent series, A Brief History of Disbelief (here on YouTube) but keep forgetting to share it ....
Is God willing to prevent evil, but not able?
Then he is not omnipotent.
Is he able, but not willing?
Then he is malevolent.
Is he both able and willing?
Then whence cometh evil?
Is he neither able nor willing?
Then why call him God?