If I was given the opportunity to make one rule with the assurance that everybody in this world would be under compulsion to obey, the rule would be LOVE EVERYONE AS MUCH AS YOU LOVE YOURSELF.

I chose this rule because as at today, the whole world is in chaos and love doesn't exist at all. There is hatred everywhere, hatred for different races, hatred for different religion, hatred for people with different ideologies etc the list is endless. But I'd everyone is compelled to obey this simple rule, then we would have successfully solved the issue of war and hatred and brought permanent peace to the world.

This is so because the absence of love, gives way for hatred and hatred leads to anger, anger to chaos, chaos to war and disaster and the presence of love on the other hand brings about an all round peace all over the world.

In a nutshell, the greatest rule that I can think of that would directly impact the world in the positive way of everyone is compelled to obey is LOVE EVERYONE AS MUCH AS YOU LOVE YOURSELF.