If you want to determine which of your qualifications can be classed as your greatest strengths, you should take into consideration:
What skills have you used effectively in the past that will work as a great example
What strength is relevant to the job you’re applying for
One of your strengths might be your communication skills but if the job description states that they favour someone who’s a great public speaker, then you should zero in on your public speaking skills and give examples of how you’ve used public speaking skills in previous jobs.