The antonyms for the word "most unexceptionable" are numerous, including unacceptable, faulty, flawed, doubtful, imperfect, questionable, unsatisfactory, deficient, and flawed. These words all represent a deviation from the ideal, a certain level of weakness or incompleteness that makes something less than desirable. Whether it be the quality of a product, the performance of a task, or the behavior of a person, these antonyms essentially represent the opposite of excellence or perfection. While there may be varying degrees of negativity associated with each of these words, they all suggest that there is room for improvement or correction.