Sharing a 'greatest' accomplishment will clue them into your values, personality, motivators, and how this personal interest fuels your dedication toward success. Why you followed this accomplishment through to completion will clue them into whether it stands apart from your professional pursuits or potentially complements their work. When you offer your example, consider why this achievement was important to you, how you went about it, and what it contributed to your life. If possible, try to present yourself as an innovator with a unique lens toward outside-of-the-box thinking.