leading has 9 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. Its one recorded opposite is following.
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.
greatest in importance or degree or significance or achievement
“our greatest statesmen”
going or proceeding or going in advance; showing the way
“we rode in the leading car”
having the leading position or higher score in a contest
“he is ahead by a pawn”
Synonyms, antonyms and the broader and narrower relations for leading come from Princeton University’s WordNet. Full sources and licences.