The antonyms for the phrase "most mind-changing" would be words that indicate a lack of impact, influence or persuasion. Some possibilities might include words such as unimpressive, unconvincing, unyielding, unchanged or immovable. These words might be used to describe a situation in which an argument or debate failed to sway someone's opinion, or in which a particular stimulus had no effect on a person's thoughts or behaviors. Alternatively, the antonyms for "most mind-changing" could be words that suggest a lack of clarity or understanding, such as confusing, perplexing, obscure, or unintelligible.