What is the most difficult question you've been asked?

That's a hard question to answer. I would have to say the most difficult question I've been asked would have to be "How can you ensure that you will be successful at this job and why should I hire you?". That was a tough question to answer and required me to really think through what I had to offer and how I could best demonstrate that to the interviewer. Update (2023-01-09): "If you could have any superpower, what would it be and why?"