What is your biggest weakness

Your key goal with this question is to not confess a weakness that could be taken by the interviewer as a negative that would disqualify you from further interview consideration. In this question you should tell them a weakness that can be interpreted as a strength at the same time, essentially turning the question into one about your strengths. This example is what you should aim for;

I believe that my main weakness is that I sometimes take on too many projects with not enough time to complete them to my personal level of quality. I have learned that I have to prioritize work loads and to set more attainable goals. Because of this I have found that I am more focused and productive in both my personal and professional lives.