you need to have a clear understanding of what the perfect candidate for the job would be and how best you match that description – more so than the other candidates. But don’t go overboard in your answer; you probably don’t know anything about the other candidates!
This sort of question generally comes towards the end of an interview so if you feel that the interviewer’s previous questions have failed to cover one of your major selling points then now is the time to speak up or forever hold your peace.