"Couthest" is a rare word these days, and even though it means "most polite or refined in manners," it is not widely used in modern English. There are, however, numerous synonyms that can be used interchangeably with this term, such as "civilized", "cultivated", "urbane", "well-mannered", "sophisticated", "elegant", "polished", "gracious", "genteel", "courteous", and "debonair". Each of these words reflects a particular aspect of good manners, but they all convey the same idea of being courteous, refined, and elegant. Regardless of which synonym one chooses, it is important to remember that exhibiting good manners and civility is always in style.