With about 300 universities and many research centers, USA is the most important country for science and research. US universities are the best the ranked research institutions in rankings like Shanghai ranking or the Times Higher Education (THES). The USA have moreover the highest number of Nobel prices in absolute number. The quality of research and education in USA is also due to the tradition of imigration of scientists from all regions of the World. Well-known is the emigration of German scientists after the Second World War.
From its emergence as an independent nation, the United States has encouraged science and invention. It has done this by promoting a free flow of ideas, by encouraging the growth of "useful knowledge," and by welcoming creative people from all over the world. The bulk of Research and Development funding (64%) comes from the private sector, rather than from taxes
See also the Science and technology in the United States on Wikipedia