The United States are the leading country in the world by nominal GDP by far. The GDP of this country accounts for more than 19 trillion USD, and that i approximately 7 trillion USD more than the second placed China, and almost five times more than than the third placed Japan.
It is worth noting that the USA's GDP is slowly but surly getting closed on by the GDP of China, which has been the country with the biggest economic progress in the world, and in fact it is the biggest producer of goods in the world, so in relatively short time China might overtake the number one spot.