There are various synonyms for the phrase "most certainly" which can express agreement or certainty. Some of the synonyms are: definitely, absolutely, surely, positively, without a doubt, unquestionably, undeniably, and undoubtedly. All of these words can be used interchangeably depending upon the context and level of assurance required. For instance, "definitely" is used when one wants to express strong assurance or belief, while "unquestionably" denotes complete certainty without any room for doubt. One can also use phrases like "for sure" or "no doubt" as synonyms, which serve to emphasize certainty and strong conviction. In all situations, one can choose from a variety of synonyms to best suit the tone and context of the conversation.