The rankings are based on economic data and a survey of 15,000 business executives and includes 142 countries. USA used to be number 1 in 2008 but has fallen to number 5. The reason is the US government debt and declining public faith in political leaders and corporate ethics. But the US productivity, highly sophisticated and innovative companies, excellent universities and flexible labor market are helping the US to remain in the top 5.
One interesting fact is that the emerging markets are climbing on the list with China at 26, Brazil at 53, India at 56, and Russia at 66.