Andreas Hoefert, chief global economist at UBS has provided a long range projection for 2025 and 2050 using purchasing power parity (PPP) and a formula that takes into account such things as capital and labour growth.
Using this technique, the United States is today's largest economy, followed by the European Union, China, Japan, India and Germany. [Wikipedia has lists of countries by PPP GDP] They also have a list with estimates projecting for the current year
But things get more interesting in 2025, when China takes the No. 1 position, followed by the United States, the EU, India, Japan, Brazil and Indonesia. By 2050, India moves into the No. 2 position, behind China. As well, countries such as Pakistan, Mexico and Bangladesh crack the top 12.
As fast as China is expected to grow by 2050, he noted India is actually expected to grow even faster. Vietnam, which is not expected to make the top 15 largest economies by 2050, also has an excellent growth story to tell, given its young, educated workforce and stable political environment.