What would it be? The restriction is that it must have been (presumably) known by a human at some point. (This should allow for more realistic options). Thus, most scientific questions are out, you're mostly restricted to things like unsolved crimes. You can include things with a partial solution if you'd like to know the full one.
Clearly my special "Are you there God, It's Me Dids" will make a lot of money- and then either I will look dumb, or more likely, the world will be thrown into chaos.