Dear Das500,
First, let us understand your question here. I feel the terms "Best Answer" and "Greatest Flaw" are both irrelevant here. Both the answers and the flaws are relative terms. My best answer could be the worst for you, and your greatest flaw may be my strongest point. Hence, your search here should be for "good answers" and not the best one.
Having settled that, let me come to my answer. I feel, as a leader myself, I must know that nothing in this world is completely flawless. Right from the source of life, the sun, to our own functional human body, everything is with a flaw or two. The sun sets and even gets clouded, the human body is so asymmetrical and changing every second.
Having realized and accepted that nothing in this world is flawless, as an interviewee, let me accept my flaws. However, I shall not term any of it as the greatest as there may be nothing as such. The number of flaws in my leadership may vary from time to time as they may be fluctuant and depend on the situations themselves.
Finally, just for your question's sake, the greatest (?) flaw in my leadership would be to think that I, being flawless, am the greatest leader of all.