conscientious has 2 distinct synonyms spread across 1 of its 2 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 unconscientious.
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.
characterized by extreme care and great effort
“conscientious application to the work at hand”
guided by or in accordance with conscience or sense of right and wrong
“a conscientious decision to speak out about injustice”
Synonyms, antonyms and the broader and narrower relations for conscientious come from Princeton University’s WordNet. Full sources and licences.