brilliant has 8 distinct synonyms spread across 5 of its 6 senses. 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
having or marked by unusual and impressive intelligence
“our project needs brainy women”
characterized by grandeur
“the brilliant court life at Versailles”
1 of the 6 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 brilliant.
Synonyms, antonyms and the broader and narrower relations for brilliant come from Princeton University’s WordNet. Full sources and licences.