The word "most unmemorable" implies something that is forgettable, bland, and unremarkable. Its antonyms or opposites could include "memorable," "unforgettable," "remarkable," "incredible," "extraordinary," "noteworthy," "distinctive," "impressive," "outstanding," "striking," and "remarkable." If you want to describe something that stands out, leaves an impression, or makes a lasting impact, you would use these contrasting words. On the other hand, if you want to express a lack of impact, significance, or relevance, you could use "most unmemorable" or other synonyms such as "plain," "ordinary," or "mediocre." Opposites can help you emphasize what you mean and convey the right message in your writing or speech.