Any city in North Korea would probably be the most dangerous city to visit especially from an American. Even though there seems to be a little less tension between North Korea and the United States, it would not be a good idea to visit there. It is not safe for you to visit that place.
Another place which is just as dangerous as the cities in North Korea would be any city in Syria. All of these places would tie for first place about being the most dangerous cities to visit. Syria is at war and you could get killed in the battles. North Korea also is dangerous because they do not like Americans and they might capture you if you came into their country.