The antonym for the word "most unsimilar" can be "most similar." When we say that two things are unsimilar, we mean that they are different or dissimilar in some way. On the other hand, when we say that two things are similar, we mean that they are alike or have some resemblance. Therefore, if we want to describe two things that are not most unsimilar, we can say that they are most similar. For instance, if two people are dressed in the same color and style of clothing, we can say that they are most similar in appearance. In summary, "most unsimilar" is the opposite of "most similar.