The phrase "is best behavior" is used to refer to someone who is displaying good manners or behaving in a polite and acceptable way. There are several synonyms that can be used to convey the same meaning. For instance, one can describe such behavior as exemplary, impeccable, admirable, commendable, or even sterling. Other suitable synonyms include courteous, gracious, decorous, refined, and tactful. These words can be used interchangeably to describe someone who is exhibiting the best behavior, whether it is in a formal or informal setting. By using these synonyms, one can add variety and depth to their writing while still effectively conveying the desired meaning.