The problem with this question is that you're being asked about your shortcomings, when your instinct, in an interview situation, is to keep your flaws as well hidden as possible. What you need to do is to frame your answer to as to give it a positive spin.

Strengths and weaknesses can be different sides of the same coin, so another way to approach this question is to think about how you overcome the potential downside of your greatest strength. For example, if you're a natural teamworker, is it difficult for you to cope with conflict or assume leadership abilities? How do you cope with this?