Tricky question. There are various sources online that rank languages by number of native speakers, number of total speakers, and number of countries in which a language is spoken.
In my opinion, I consider the six languages of the United Nations (Arabic, Chinese, English, French, Russian, and Spanish) the languages with the broadest overall utility. But there's no objective scale for the usefulness of a language.
Of course, the "top" language is always the one you enjoy speaking the most :)