Haha... the lottery question would have been the one... but to know if there are aliens amongst us, that would be absolutely mind-blowing if there were... and somewhat disappointing if there wasn't!
I think someone also mentioned asking if they could trust someone! I think given that people are fickle and changeable, the answer that they give might be truthful for the moment, but might be different an hour later!
But if we all asked the lottery question and all won - the winning would be so small :) Another thing to be better left to the mysteries of life...