Antonyms for the word "middle-most" include "outermost," "extreme," "peripheral," "edge," "border," "marginal," and "endmost." These words all refer to something that is at the edges or outermost part of a particular object or location. For instance, an "outermost" object is furthest away from the center, while a "peripheral" object is located on the edge or outer limits of something. "Marginal" and "border" both refer to something that is not at the center, but rather located on the fringes. Using these antonyms can provide clarity in speech or writing and help to avoid ambiguity or confusion.