It really doesn't matter which language you choose. Selecting a language you study will be more effective if you are interested in that language and the cultures that it represents.

Think of reasons you want to learn a specific language:

family heritage;

useful foreign language in your country (e.g. Spanish in the United States or French in Canada);

desire to travel to places that speak it.

Fin out where all known living languages are spoken in the world: http://www.ethnologue.com/

Find out the linguistic composition of the United States: http://www.mla.org/map_main