ultimate 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 proximate.
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.
furthest or highest in degree or order; utmost or extreme
“the ultimate achievement”
the finest or most superior quality of its kind
“the ultimate in luxury”
1 of the 3 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for ultimate.
Synonyms, antonyms and the broader and narrower relations for ultimate come from Princeton University’s WordNet. Full sources and licences.