If you choose to learn a new language, I'd suggest to do it for the joy of learning and discovering something new. The fact that you ask which one is the most valuable shows that you don't feel you need another language for any practical purpose. Don't look for the easiest or "most valuable" (whatever that might mean). Look for one you'd enjoy.

You can check out some online language learning communities such as UniLang or HowToLearnAnyLanguage for a lot of discussion on this.