The antonyms for the word "most nascent" would be words that describe something that is already fully formed or established. Some possible antonyms could include "mature," "ripe," "complete," "settled," "steady," or "experienced." These words suggest a greater degree of development and stability, as opposed to the more tentative and uncertain connotations of "nascent." Other antonyms could include words that describe a decline or deterioration from a previously higher state, such as "declining," "degenerating," or "waning." All of these antonyms convey a sense of finality or certainty, in contrast to the potential for growth and change implied by "most nascent.