The 20th century has often been called The American Century. The US did indeed do well over this century. They became the foremost economic and military power. However, when you look at GDP per capita (a measure of the wealth of a nation), you find that they don't exactly stand out. For example, France and Germany didn't grow their GDP must slower than the US over this period. In fact, they started out a few percent behind the US and ended up about 30% lower. Ok, but they were both nearly destroyed by world war. Germany lost both wars and suffered a period of hyperinflation in between. They both suffered from stagnant population growth unlike the US which grew population rapidly. In addition, the US had more abundant natural resources. Canada basically tracked the US. No one calls it the Canadian Century.
However, If GDP growth is your metric for naming centuries, you would have to choose Japan. Japan is basically equal to the US in terms of present day GDP per capita. But in 1820, GDP per capita was half that of the US. In 1900, it was 28% of the US. In 1950 (after losing WWII to the US) it was only 20% of the US. Now they are tied. So the GDP growth crown for the last century and especially the past 50 years has to go to Japan.
The darlings of today's investment crowd, China, India and Brazil have lagged miserably. In fact India has hardly grown real GDP per capita at all in almost 200 years. Brazil has done OK, since 1900 but was quite far behind.
