An interviewer will ask this question to understand how you define success and see how goal-driven and ambitious you are. To answer this question, you want to think of all of your professional accomplishments and narrow it down to the one accomplishment you are most proud of. While you might want to share achievements that are not professionally related, such as having children, climbing Mt. Everest, or finishing your first marathon, the interviewer would rather hear about professional or academic accomplishments.