Basically the North is a totalitarian state in which one family has retained absolute control of the state and its population. They have prioritised military programmes and their nuclear ambitions above the needs of their people and thus they have one of the worst economies and their people are often starving and diseased.
Recently they have also been arming Russia to help with their war of aggression against the innocent people of Ukraine.
All of this is in contrast to South Korea which is a democratic country with one of the world’s strongest economies which provides for a very good standard of living for its population.
There is much more, I’m sure, but these are the first things that pop to mind and the reason why the North is not the best Korea, and infact the South should be considered the Best Korea.