In one word: English
If you speak English well already, then: Spanish, Mandarin, French, Arabic or Russian
If you need to learn a language for work, then: learn the language of the country where you will be working, or the country at which your work will be aimed at.
If you'd just like to learn a language because you like learning languages, then: pick whichever language you are attracted to emotionally.
Learning languages has always been very popular. At least in countries where learning foreign languages is necessary. The most important language from where I come from to learn is English and if you would ask me, which language should you learn or work on for economic or career reasons, I would not hesitate one moment and say, you should learn English. The languages you should choose to learn however often do not depend on how much money you can make knowing them or by how much better your career prospects will be. Many factors are very individual and greatly depend on the reason why you'd might want to learn the language more than anything (obviously).
The reason
There are several reasons why you might want to learn a foreign language and usually it's one of the following: