The antonym of "most unalloyed" would be something that is mixed, impure or diluted. Words that could be used against it include adulterated, contaminated, hybrid, alloyed, blended or impure. To explain this better, we could take an example of ethical behavior where terms like dishonest, corrupt or insincere can be considered antonyms of "unalloyed." Similarly, the taste of a drink can be "unalloyed" if it is pure and unadulterated. Its antonyms in this case could be mixed, diluted or even artificial. Thus, antonyms for the phrase "most unalloyed" would mostly comprise of words that convey a deviation from purity or authenticity.