I picked English because it was the best of the 2 choices given but you really ought to consider Spanish. It is spoken in Spain, the central American countries, South American countries, and many of the islands in the Atlantic and Pacific. And once the illegals take over the U.S...... ;)

Saluton! Mi peti cxiuj homoj lerni Esperante.

.... I suggest Esperanto, it is very simple to learn. You can learn it about 10 times faster than Spanish in my experience. Studies in Germany show that it is easier to learn than English ^1 (Edit: Dead link). English is just a situational dominant language, like Mandarin (There is no Language called Chinese) will be in the future. Esperanto is a politically neutral language.

Another thing is that Chinese students are learning it right now as a option other than English, and the EU is looking at it as an alternative to teaching other languages (UK already does so in a limited context). It is becoming increasingly common, I just say it is a matter of time.

I would say English seeing as how most places that don't dominantly speak it are required to learn it.

I speak English so my answer is a prejudiced one but it would be great if everyone did speak English so we could all understand one another.