Antonyms for the phrase "most supposable" can include "impossible," "unlikely," and "implausible." These words suggest that the likelihood of a particular event or scenario occurring is low or non-existent. "Inconceivable," "unimaginable," and "unfathomable" are also antonyms for "most supposable." These words suggest that an event or scenario is beyond the realm of possibility or comprehension. In contrast, synonyms for "most supposable" might include "probable," "likely," or "reasonable." These words suggest that an event or scenario is possible, logical, or expected based on existing information or evidence. Antonyms can help individuals to better understand the nuances of language and communicate with greater precision.