fundamental has 9 distinct synonyms spread across 4 of its 5 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.
being or involving basic facts or principles
“the fundamental laws of the universe”
far-reaching and thoroughgoing in effect especially on the nature of something
“the fundamental revolution in human values that has occurred”
any factor that could be considered important to the understanding of a particular business
“fundamentals include a company's growth, revenues, earnings, management, and capital structure”
Synonyms, antonyms and the broader and narrower relations for fundamental come from Princeton University’s WordNet. Full sources and licences.