This is a question about self-awareness. Everybody has strengths and weaknesses, so don’t be afraid to admit that you are not perfect! However you need to make sure that you don’t tell the interviewer that your biggest weakness relates to a key skill required for the job.
A good approach is to admit a real weakness (which does not impact on the job), then describe how you overcome it by using strengths which are relevant to the job. By talking about how you manage your weakness, you can finish your answer on a positive note. You could, for example, refer to overcoming nervousness at public speaking through preparation and organisation.