The antonyms of "most unbacked" are the opposite words that could replace the phrase's meaning. These words could be "fully supported," "heavily defended," "substantially reinforced," and "thoroughly backed." If we take the context of the phrase, we could find the suitable antonyms. For example, if "most unbacked" is used for a statement, the antonyms could be "evidently proven," "undeniably supported," "credibly substantiated," or "convincingly backed up." In contrast, if the phrase is used for a person or an organization, "reliably assisted," "dependably supported," "assuredly backed," or "unsurprisingly reinforced" could be the antonyms.