Synonyms for Fantastic:

adj.

enormous (adjective)
prodigious, tremendous, great, stupendous, huge.
exaggerated (adjective)
sensational.
exceptional (adjective)
outlandish, fabulous, phenomenal, excellent, noteworthy, striking, unprecedented, sensational, unusual, exotic, superior, exceptional, miraculous, extraordinary, peculiar, notable, outstanding, remarkable, wonderful, novel, Wonderous, marvelous.
fictitious (adjective)
invented.
illusive (adjective)
phantasmagoric, hallucinatory.
imaginary (adjective)
imaginary, dreamy, inventive, chimeric, hypothetical, speculative, suppositional, conceptual.
notional (adjective)
notional.
strange, different; imaginary (adjective)
fictional, implausible, out of sight, hallucinatory, preposterous, quaint, erratic, artificial, odd, extreme, extravagant, whimsical, unbelievable, weird, ludicrous, irrational, peculiar, singular, incredible, absurd, far-fetched, eccentric, capricious, unreal, freakish, foreign, ridiculous, comical, illusive, grotesque, outlandish, exotic.
suppositional (adjective)
theoretical, hypothetical, Prophesied, suppositional, presumed, imaginary, speculative, conjectural.
unbelievable (adjective)
implausible, farfetched, preposterous, incredible.
wonderful, excellent (adjective)
sensational, superb, marvelous.

n.

unbelievable, attractive, restful, relaxing, enjoyable, nice, pleasing, cheerful, pleasurable, pleasant. mercurial, unpredictable, change, temperamental, fickle, variable, volatile, unsteady, unstable, uncertain, inconstant, ticklish, inconsistent, changeable, continue. astounding, astonishing, stupendous, amazing, prodigious, good. fictive, made-up, fictional, invented, fictitious. hot, ripping, cool, keen, glorious, superb, swell, super, splendid, nifty, groovy, neat, great, dandy, divine. fantastic (noun)
phenomenal, fantastical, grotesque, terrific, marvelous, antic, strange, rattling, marvellous, extraordinary, wondrous, wonderful, howling, unreal, unusual, tremendous, unrealistic.

Other synonyms:

invented, astounding, fictional, nifty, fictive, groovy, ripping, far-fetched, notional. extreme, astonishing, fictitious, unpredictable, chimeric, mercurial, stupendous, amazing. prodigious, variable, capricious, inconstant, temperamental, made-up, inconsistent. ticklish, volatile, changeable, fickle, unbelievable, freakish, dandy. great, glorious, unsteady, unstable. hot, divine, splendid, cool, neat, keen. uncertain. excessive
extravagant.

Usage examples for fantastic

  1. Lance had listened with profound attention; and the shadowy, fantastic outlines of the narrative were so extraordinary, that he was at first too much astonished and perplexed to know what to think or say about it. – True and Other Stories by George Parsons Lathrop
  2. You can picture to yourself what a unique and fantastic sight this must have been! – In the Courts of Memory 1858-1875. by L. de Hegermann-Lindencrone
  3. For what seemed ages the fantastic figure stood there. – The Passing of Ku Sui by Anthony Gilmore