This is a tough question. A good way to interpret it is to find an area of your job that you are still improving on. You want the interviewer to know that you're aware of your weakness and are working on improving/correcting it. This is a time when you can show that you are constantly taking steps to improve on your skills. Ideally, you should mention a small weakness like being too critical of yourself or letting yourself get bogged down by small details. Once you mention this weakness, you should follow up with ways you are improving on it. At the end of the day, you want to choose a weakness that isn't detrimental to your interview - it shouldn't harm your chances of getting the job.