The quality debate about scientific merit aside, if the U.S. wants to remain "the best", then we need to incentivize the best scientists and researchers in other countries to come here and become U.S. citizens. A lot of famous physicists, mathematicians, and logicians have done this over the past 100 years.
Unfortunately, the attractiveness of becoming a U.S. citizen is waning because (1) current U.S. policies on science are extremely unfavorable to foreign researchers and (2) other countries are making it much more desirable to remain a citizen there than to become a U.S. citizen (this is a net win for humanity though)
I think (1) is in part due to a culture in the U.S. that really just doesn't care about science anymore. When I was in grad school a few years ago, most of my peers were from other countries despite the fact that the majority of undergrads at the same school were born in state. So in effect, the grad students would get advanced degrees in the hard sciences and take that knowledge back home with them, whereas the U.S. students would get a B.S. in literature, advertising, or business (generalizing of course).