Pick a weakness that is not a core skill for your success as a medical student. You can be candid in your answer, recognizing that you aren't great at something and acknowledging your need to improve. Be sure to have an action plan in place for improving on this weakness.
Perhaps you are watching TED talks to gain skills in a particular area, reading the latest-and-greatest book on the subject, or maybe you are taking a seminar at a nearby community center. We are all human with our own weaknesses, so don't be afraid to share yours!