What is the world's greatest challenge?

That probably depends on who you are, since different things are challenging to each of us.

Having a loved one die might be top on the list of things that would be greatly challenging for all people.

Living a happy life, while having a sustainable income and a good social life.

Other possibilities:

• Learning the Navajo language is supposed to be incredibly difficult.
• I'm sure some people would see winning a medal at the Olympics as an ultimate life challenge.