diligent has 1 distinct synonym 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 negligent.
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.
quietly and steadily persevering especially in detail or exactness
“a diligent (or patient) worker”
characterized by care and perseverance in carrying out tasks
“a diligent detective investigates all clues”
Synonyms, antonyms and the broader and narrower relations for diligent come from Princeton University’s WordNet. Full sources and licences.