By asking this question, your goal should be to find out what quality the interviewer values most for this position, assess whether you possess this quality, and then speak further about how you will meet this need.
For instance, perhaps the interviewer says that the most important quality to possess in this role is reliability. You can then begin to speak about how you have been reliable in the past or with your most recent employer. Perhaps you stayed late when necessary, checked your emails after hours when you knew a client would be responding, or maybe you would bring coffee for the team when they were working late on a project.
If the quality that the interviewer mentions are something you are not overly versed in, you can speak about how you are strengthening that area. For instance, if the essential quality is to be amazing in Excel, you could let the interviewer know that you are an intermediate user and you are taking coursework this month to reach expert-level status by the end of the year.
Whatever the interviewers' response, be ready to express how you will bring this vital quality to the company if you are the successful candidate.
