GDP PPP seems to be pretty useless for such comparisons. Nominal GDP should be used. According to the following three sources: * International Monetary Fund,

GDP PPP seems to be pretty useless for such comparisons. Nominal GDP should be used. According to the following three sources: * International Monetary Fund, * World Bank * CIA World Factbook the crown of the world's largest economy belongs to the European Union (and has belonged for some time now).