"What is your worst defect?" is a fairly rare form of the common interview question "What is your biggest weakness?" The hiring manager may use the word "defect" instead of "weakness" in an effort to get you to share a more personal example. Careful planning and practice can help you prepare for this type of question.
Rephrase the Question
If you do get the question "What is your worst defect?" rephrasing the question before answering is a good place to start. This approach helps you frame your response more effectively. You might start with "I don't often think of myself as having defects, but like all people, I'll certainly have areas that could use improvement." You don't want to come across as condescending or overly critical of the question, but a subtle reshaping of it allows you to more adequately offer a modest weakness rather than your "worst defect."
One of the reasons a hiring manager asks the weakness question is to get a sense of you as a person. If you say "I have none," he will likely take you as insincere or lacking humility. Some career coaches suggest flipping the weakness by saying something like "I tend to get overly involved in my work to the point that I spend a lot of energy on each task." This is a bit too over-the-top, scripted and disingenuous for the tastes of many hiring managers. A more genuine and humble response, such as "I tend to develop clutter on my desk that can get overwhelming over time," tends to work better.
More to the Story
Being sincere and sharing a weakness aren't the only important elements of a good strategy. You need to consider how your weakness will impact the hiring manager's perception of you as a potential employee. Saying "I get very nervous when speaking in public" may be genuine, but it won't serve you well when you interview for a job in management or teaching, which both require strong public speaking skills. For a supervisory management role, a genuine, but less-risky option is "I tend to emphasize relationship-building and development of my employees, which sometimes challenges my abilities to schedule time for administrative tasks."
Other Strategic Considerations
Before the interview, you should review the list of desired traits in the job posting. Similarly, list your weaknesses. Select one that doesn't appear important to the needs of the position. Offer a specific, but underwhelming trait or behavior that won't set off the alarms of the hiring manager. An actuary typically needs strong analytical skills and attention to detail, along with business sensibility. Creativity, though nice, isn't a major requirement for actuaries. Thus, if you are more analytical than creative, you might say "My work demands that I place heavy emphasis on precision and details, which sometimes precludes me from engaging in creative activities."