fewest has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is most.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(superlative of `few' used with count nouns and usually preceded by `the') quantifier meaning the smallest in number
“the fewest birds in recent memory”
Synonyms, antonyms and the broader and narrower relations for fewest come from Princeton University’s WordNet. Full sources and licences.