Spanish. Spanish is number two spoken behind English in the US. Of course there are pockets around the country where a different language is number two. Tarpon Springs, FL I assume it is Greek. Dearborn, MI it is likely Arabic. North Vermont it might be French.
Exceptions to Spanish being the best language to know would be if you are working in a specific business that deals with a specific country. Like for some Chinese might be most useful.
If you want to work for the CIA Arabic and Farsi were in need, I assume that is still the case.