Yes the United States of America is indeed the most powerful country in the world. It controls Europe and most of Asia with it's military might. With nato and the UN backing up the states no matter the situation. The only true opponent to their military and economic might would be the Russian Federation.
As I mentioned earlier the People's Republic of China is a possible economic competitor to the U.S, however be reminded that China has a powerful economy only because they signed an agreement with the United States that products developed in China can be imported to the US without tax. Without this agreement everything would be made in the USA and not in China. However it is almost impossible not to mention the Russian Federation in this Conflict. They are probably the only country in the world that can match the United State's military might.
Actually China did have a pretty poor economy in the 20th century and was way behind in technological advancements. They do have a good economy but remember without the agreement between the U.S. and the People's Republic of China, the Chinese economy would have been no match to the U.S economy since everything would have been made in the US. So think about it. Yes it is true that China has a great economy, however their currency is also dependent on the dollar since it is the Chinese Yuan is pegged to the American Dollar. On top of this what good is an economy to a massive, global, heavy armed, high-tech armada of the United States of America.