The antonyms for the phrase "most pretenseless" would be words that describe someone or something that is insincere, deceitful, or disingenuous. Some antonyms for the phrase may include "phony," "fake," "pretentious," "artificial," "dishonest," "deceptive," "hypocritical," and "pretending." These words all suggest that the person or thing in question is not honest or authentic, and that they are presenting themselves or acting in a way that is not genuine. In contrast, the phrase "most pretenseless" implies a pure, simple honesty and authenticity, devoid of any kind of deception or artifice.