If you want to know what language has the most speakers, it's definitely Mandarin Chinese, followed by Hindi, English, and Spanish. (It is unclear whether English or Spanish has more speakers.)
But your words were, most WIDELY spoken, and I'd have to say English, as it is spoken in 105 countries (83 officially). By this definition, French and Spanish are the runners-up.
That's not to say Chinese isn't huge. Mandarin's 874 million speakers (approximate) are enough to top the charts, but that doesn't even take into account non-native Mandarin speakers or any of the other dialects. The non-natives put Mandarin just over a billion, and the total number of Chinese speakers is approximately 1,343,755,000.
Edit, re Positivemindattitude: It is true that Arabic is a widely spoken language. Regarding the number of speakers, it would come just after the languages I mention in my first paragraph. It is also the official language of 26 countries, five more than Spanish, but Spanish is spoken unofficially in many places. French still beats both by a long shot (albeit not by population).