The choice depends on your reason for learning it. If you are wanting to have more options in business, you need to know which languages are used around the world for whatever business you are in. Good worldwide languages at present - those that cover a lot of different territory - would be Spanish, French, Mandarin Chinese, Japanese, Russian, Arabic, and Hindustani.
If you are learning it for pleasure, you might want to consider whether you want something easy or challenging. For the easy choice, select a language related to your own mother tongue (for example, English-German-Dutch-Norwegian-Swedish-Danish would be one group; Spanish-French-Italian-Portuguese-Romanian would be another; Slavic and Semitic languages would form other groups).
For a challenge, do the reverse and choose a language that is not related to your own. Some people even like to learn languages that few people know, in an attempt to help preserve languages that are dying out.