If you are looking for a second language to learn, the answer depends on your needs. For people who are not native English speakers, learning English is perhaps the best choice. English is arguably the ‘official’ universal language, and many businesses require a certain level of English for an employee to progress.
If you already speak English fluently, you might want to take a look at learning Mandarin Chinese, Spanish, Hindi/Urdu, Arabic, Bengali, Portuguese, Russian, and Japanese. These are the languages with the most native speakers in the world. If you simply want to learn a language for the love of it, try French and Italian – arguably two of the most beautiful languages.