The word "un-fantastic" means not good or ordinary. Its antonyms are words that describe something amazing, extraordinary, and exceptional. The opposite of "un-fantastic" is "fantastic" which means excellent, breathtaking, and remarkable. Other antonyms for "un-fantastic" include "outstanding," "phenomenal," "marvelous," "splendid," "wonderful," and "spectacular." These words describe something that is beyond the norm, exceeding expectations and leaving a lasting impression. When you want to express that something is truly remarkable, you can use these words as an antonym for "un-fantastic.