No one likes to answer this question because it requires a very delicate balance. You simply can't lie and say you don't have one; you can't trick the interviewer by offering up a personal weakness that is really a strength ("Sometimes, I work too much and don't maintain a work-life balance."); and you shouldn't be so honest that you throw yourself under the bus ("I'm not a morning person so I'm working on getting to the office on time.")

Think of a small flaw like "I sometimes get sidetracked by small details", "I am occasionally not as patient as I should be with subordinates or co-workers who do not understand my ideas", or "I am still somewhat nervous and uncomfortable with my public-speaking skills and would like to give more presentations and talk in front of others or in meetings." Add that you are aware of the problem and you are doing your best to correct it by taking a course of action.