What was your greatest accomplishment as of yet outside of work?
Avoid clichés and choose something that reveals a new side to your personality.
Good areas to consider are sport, music, cooking and volunteering. Give an example that allows conclusions to be drawn about your personality. Your interviewer can then consider how these character traits might also apply to your professional life.
Have you run a marathon in under 3 hours? This would be a perfect example as it shows that you are capable of thinking long-term and being dedicated and disciplined.