My favorite interview question is “what has been your biggest mistake, and what did you learn from it?”
Most candidates stumble on this. If this is hard to answer you either have not been in a position of responsibility or you lack insight and do not reflect and learn from your mistakes.
I wonder how the presidential candidates would answer this question?
My other favorite questions:
If you had all the money you could ever need, what would you do with your days? (It clarifies what motivates a prospect.)
How is your closet organized? (the enthusiasm of the response more than the repsonse itself tells me how organized they are)
Give me an example of a time when you went beyond the call of duty? (This is a tough question for most, often because a great candidate may be unaware that they have done something exceptional.)