The antonyms for the word "most conjecturable" would include words and phrases like "certain," "definite," "undeniable," "established fact," "proven," and "verified." These antonyms imply a high level of confidence and conviction in the truth of something, as opposed to conjecture or speculation. Other antonyms might include words like "impossible," "implausible," "absurd," "unbelievable," or "unfounded," which suggest that an idea or claim is not based in fact or reality. When choosing antonyms for "most conjecturable," it is important to consider the context and tone of the sentence to ensure that the phrasing is appropriate and effective.