The U.S. has kept the world order such as it is, with no wars on the scale of  the world wars, for nearly a century.
During that time it freed hundreds of millions if not billions of people rather than have a lot of them risk their lives and that of their children trying to escape from their communist and dictator captors.
How many countries, other than your own, do you think reluctantly accepted the U.S. as world cop, and prospered in large part in not providing an adequate national defense?
When is the last time there was not a dominant country on the planet?
Do you think with no world cop that the Strait of Hormuz shipping lane would be kept open on the honor system?
Who do you suppose is keeping China from claiming even more of the south China sea? 

A response of 'none' when the choice is the U.S., China or Russia for world cop, e.g. the most dominant country, 
is not an answer because the context of the question is the real world. Not the way you imagine it.