From Longman Dictionary of Contemporary Englishthe single biggest/greatest etcthe single biggest/greatest etcused to emphasize that you are talking about the one thing that is the biggest, greatest etc Cigarette smoking is the single most important cause of lung cancer. Tourism is the country’s single biggest earner. → single
Examples from the Corpus
the single biggest/greatest etc• For the single greatest cultural movement of the twentieth century is the rise and global hegemony of black music.• The survey showed that consumer concern about the economy was the single biggest factor affecting the building business in 1993.• Drug overdoses have become the single biggest killer among the city's young people.• It was the single greatest revelation of his religious life.• It represented the single biggest step towards the creation of the international air agreements of today.• This is the single biggest thing we could do to reduce costs. 