"El peor del mundo."
Translation:The worst in the world.
I had "worst in the world" but changed it to "worst of the world" because duo is often unreasonably picky about using the exact same article in translation. Sometimes you just can't please that finicky green bird.
That doesn't sound right in English. "The worst in the world" is more typical
Perhaps the Duo programmer is teaching us an expression used in real Spanish. In Spain, I often saw advertisements like "The best bank account en todo el mundo" -- which always gave me a smile. My impression is that this is sort of idiomatic, an intentional exaggeration to emphasize your opinion.