effort has 15 distinct synonyms spread across 4 of its 4 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.
earnest and conscientious activity intended to do or accomplish something
“made an effort to cover all the reading material”
use of physical or mental energy; hard work
“he got an A for effort”
a series of actions advancing a principle or tending toward a particular end
“he supported populist campaigns”
Synonyms, antonyms and the broader and narrower relations for effort come from Princeton University’s WordNet. Full sources and licences.