The best way to answer is to list your actual strengths. There are a great number of interview tips online at career portals. This is the easy question, it is answering what are your weaknesses that can get you in trouble!
I would think that these two should be way up on your list of strengths.
INTEGRITY. Integrity means that you are honest and truthful in what you say or do. You put honesty, sense of duty, and sound moral principles above all else.
DEPENDABILITY. Dependability means that you can be relied upon to perform your duties properly. It means that you can be trusted to complete a job. It is the willing and voluntary support of the policies and orders of the chain of command. Dependability also means consistently putting forth your best effort in an attempt to achieve the highest standards of performance.