What is the world's best question?

There is no such a thing as the world's best question :)

I don't know if there is a such a thing like that. But you can start with this question: "Who Am I?". Knowing yourself is the best thing you can do to yourself. Reinforce this by asking your family, friends, colleagues, mentors or even your enemy. But these are their impression to you and at the end of the day, it is you who will answer it.