Look at this question in the context of the essays overall. There’s a goals question (essay 3), and a leadership question, which will likely discuss a work experience. With this question, the adcom is looking to get to know you as a person, beyond work. Also, “most important” really demands a substantive answer (some people can answer such a question with humor – Stanford has always had a similar question and I wait for the day someone will answer “my dog” – but you’d need just the right touch to pull it off!).
I suggest discussing something concrete if possible, and
root the discussion in your actual experience. For example, reading about someone’s amazing grandmother is usually much more interesting – and more revealing about them as a human being—than reading about someone’s discourse on the value of integrity. The latter may be sincere, but will probably be boring… On the other hand, if you were to answer “integrity” and then use the bulk of the essay to present compelling actual examples of why it matters so much to you, it could indeed work well.
The key is, to look at your life, answer with sincerity, and build your essay primarily out of anecdote/experience.