This is a challenging question -- as if you have no weaknesses you are obviously lying! Be realistic and mention a small work related flaw.

Many people will suggest answering this using a positive trait disguised as a flaw such as "I'm a perfectionist" or "I expect others to be as committed as I am." I would advocate a certain degree of honesty and list a true weakness. Emphasize what you've done to overcome it and improve. This question is all about how you perceive and evaluate yourself.