Job application question: What is your greatest success to date & why?

You just wrote it! Employers want an answer that sticks out, not some rote answer that you think they'll want to hear. Tell them about your life accomplishments so far, you nailed it! My essay to get into college wasn't about academics, it was about surviving a car accident and the perspective on life it gave me. They said it was impressive. So go with your life, that'll be golden.