I have gotten this question and it seemed that the interviewers really liked my answer. First of all, confidence is just as important as being humble, so I explained how I had prepared for medical school and how this preparation had made me confident that I would excel in medical school. To stop there would probably be a wrong answer since no one is perfect, whether you think you are or not. I then went on to explain that while I do not believe that I have any detrimental or glaring weaknesses, there are various things that I could work on in order to improve. At this point just continue to be honest and talk with them about things you need to improve on, whether it be study skills, time management, money management, etc. Like someone else said, just make sure to not say anything that will truly damage you, such as:
"My weakness is that I'm really not very good at sciences"
or something worse that will not only get your application rejected before the interview is over but they will call other schools and tell them not to interview you, like:
"I really do not get along well with people, especially those of other races. In fact, I hate people"
Keep your weakness that you talk about marginal, and BE SURE to explain how you plan to improve.