The opposite of "un-steadiest" is "steadiest," which refers to something that is not shaky, uncertain, or unstable. "Stable," "firm," and "fixed" are some of the antonyms of "un-steadiest" that describe something that is balanced, consistent, and reliable. Another antonym is "secure," which means something is protected from harm or danger and is unlikely to waver. "Strong" and "dependable" are also antonyms of "un-steadiest" that describe something that is sturdy and trustworthy. Using these antonyms in writing can add clarity and specificity to the description of something that is not unsteady.