We can divide this distinction into three categories: language with the most native speakers, language with the most total speakers, and the most WIDELY spoken language (this is the important one).
Mandarin Chinese has the most native speakers of any language on earth, with nearly 990 million.
ENGLISH, on the other hand, is both the world's most widely spoken language and has, by some estimates, almost 1 billion more total speakers than Mandarin. High estimates for the number of total speakers of English range from 1.4 to 1.8 billion.
English is also the only language in the world that has official status in at least one country on every continent except Antarctica. Mandarin, on the other hand, only has official status in the People's Republic of China, Taiwan and Singapore. Of these three nations, it is only the vernacular in the PRC, Taiwan speaks a form of Min Nan as its native language, and Singapore has a great mix of tongues.
When discussing the most widely spoken languages on earth, I would not even place Mandarin in the top 5. English, Arabic, Spanish, French, Portuguese, Russian, and even Swahili (if you'd like) are more widely spoken than Mandarin.
Student of linguistics