Don't put yourself down
This is in fact an unfair thing to ask. You are under the microscope in an interview situation, why should you have to concede ANY weakness? Why voluntarily put yourself down? But you have to answer, don't you? Or be rated as uncooperative and rude.
But be careful. Any career answer is going to express negative. So take control of the situation and share a human response - your biggest weakness is CHOCOLATE CAKE, or any substitute you prefer. It's a harmless answer to a potentially lethal question.