最差(zuì chà) - worst (poorest in quality) in Mandarin

最差 (zuì chà) is 最 (zuì), "most", plus 差 (chà), "poor, lacking, falling short" - worst in the sense of poorest quality or performance. 他的成绩是班上最差的 (tā de chéngjì shì bān shàng zuì chà de), "his grades are the worst in the class", is exactly the kind of ranking this word is built for: exam results, service, a product, a harvest, anything being judged on how well it performs.

This word list also carries 最坏 (zuì huài), glossed the same way in English, but the two are not interchangeable in Mandarin. 最差 (zuì chà) ranks quality and performance; 最坏 (zuì huài) describes bad circumstances, outcomes or character. A "worst restaurant" for bad food and service is 最差的餐厅 (zuì chà de cānguǎn); a "worst day" of bad luck is 最坏的一天 (zuì huài de yī tiān), not the other way round.

差 (chà) on its own is worth knowing too: 差不多 (chàbuduō), "almost, more or less", and 差点 (chàdiǎn), "nearly, almost", both build on the same root sense of falling short of a mark.