Obama media has stated that Obama told Trump that NK is the biggest problem he will face.

Perhaps the main reason it is a problem is because of lessons learned from the Cold War. NK is developing the technology to enter the world into that same situation. The other countries mentioned are not. The reason the Cold War type situation is such a problem is that because of what is known about how things happened, a good case can be made that it was by luck that the world didn't get blown to bits in the Cold War.