What's the most practical language for students to learn?

Recently, Senator Barack Obama caused a stir when he told a crowd that although immigrants should learn English, "you need to make sure your child can speak Spanish." Though some objected on principle to his call for bilingualism, it raises an interesting point. Is Spanish, still overwhelmingly the most popular foreign language taught in U.S. schools, the best choice? Is it more practical than French, spoken in twice as many countries from Canada to the Congo? China's powerhouse role on the world stage has made Mandarin Chinese an increasingly popular choice for students. Speaking Arabic, Hebrew, and other key languages can also open doors. In today's ever-shifting international landscape, which foreign language should take priority in schools?