The opposite of "most unfastidious" can be described by various antonyms, depending on the context. A few possibilities include "meticulous," "picky," "exacting," "fussy," and "finicky." These adjectives imply a high level of care and attention to detail, often indicating a preference for things that are orderly and precise. Conversely, "unfastidious" suggests a lack of concern over details and a greater tolerance for messiness or imperfection. Ultimately, the choice of antonym will depend on the precise meaning of "most unfastidious" in a given context, as well as the intended tone and emphasis of the sentence.