The antonyms for the word "most mythomaniac" are honest, sincere, truthful, reliable, and credible. These words represent the opposite of someone who is a mythomaniac, which refers to a person who constantly embellishes or fabricates stories, exaggerating their experiences or achievements in a way that is inconsistent with reality. An honest person, on the other hand, is someone who tells the truth, even when it is hard to do so. A sincere person is authentic and genuine, communicating their true feelings and intentions. A reliable person is trustworthy and consistent in their behavior, while a credible person is deemed to be reliable and believable. All of these characteristics stand opposed to the behavior of a mythomaniac.