Now, "best" is a very subjective term. In what manner are you talking about? The most useful languages? The most fun languages? As for the most useful languages, it is still a very subjective way to group things. What makes a language more useful than other? The number of native speakers? The number of people who learn said languages? Native Speaker quantity-wise, Chinese and Arabic would come out on top, but Chinese and Arabic are rarely spoken outside of their geographical areas and thus may lose the value of having more native speakers. Learner quantity-wise, English and Spanish would (probably, I didn't really fact check for this) come up on top. While I definitely agree with English being a very useful language, I'm a bit doubtful about Spanish. Studies have shown that Spanish is not so valued in the workplace (people who are bilingual in spanish and english only have a 1% higher income than english-only speakers), so its value seems to be exaggerated. Personally, I vouch for learning uncommon languages, as those languages are much less commonly studied and can really open your eyes to a different point of view.