fantastic has 13 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.
extraordinarily good or great; used especially as intensifiers
“a fantastic trip to the Orient”
existing in fancy only
“fantastic figures with bulbous heads the circumference of a bushel”
1 of the 5 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for fantastic.
Synonyms, antonyms and the broader and narrower relations for fantastic come from Princeton University’s WordNet. Full sources and licences.