Craft your story using the SAR model [situation, action, result], linking the results back to the overall achievement.
I once asked someone this question and she told me that moving to university was her biggest achievement as she missed her family terribly. Find your perfect job. Check out the video below for my tips for answering this question. Some career coaches and recruiters will advise you to always give a professional achievement.
You could even choose an achievement from your personal life which stresses your workplace values. Takes responsibility for delivering on intended outcomes. Picking out figures and statistics, such as percentage growth or client growth, is a surefire way to show how you can go above and beyond requirements.
It was such a rush to come to work and see people getting trained to do something that I helped develop. Unexpected: were you unprepared but still able to obtain a result?
I recognised that I needed to be more confident when speaking at team meetings so I could contribute to discussions of issues as well as put ideas clearly. I get this, I really do, but she had graduated 15 years earlier and I was hoping for something more recent and relevant than that.