I'm not sure why interviewers ask this question; your resume and experience should make your strengths readily apparent.

Even so, if you're asked, provide a sharp, on-point answer. Be clear and precise. If you're a great problem solver, don't just say that: Provide a few examples, pertinent to the opening, that prove you're a great problem solver. If you're an emotionally intelligent leader, don't just say that: Provide a few examples that prove you know how to answer the unasked question.