The opposite of "most adopted" can be expressed using several antonyms depending on the context. If we're talking about animals, "least adopted" would be an appropriate antonym. On the other hand, if we're talking about children or products, "least popular" or "least favored" would be better options. Alternatively, one could use "rarely adopted" or "unpopular" to convey the opposite meaning. In any case, the antonym for "most adopted" will vary depending on the situation, but the underlying idea is always the same: It is the opposite of the thing that is most widely accepted or chosen.