The phrase "most quicksilver" implies something that is highly changeable or volatile. Some antonyms for this term could be steady, stable, or constant. Such words suggest that whatever is being described is reliable and predictable, rather than erratic or unpredictable. Other antonyms might include sluggish, slow, or languid, indicating that the thing in question moves more slowly or lacks quickness altogether. Similarly, some synonyms for "most quicksilver" could include mercurial, unpredictable, or fickle. Ultimately, the choice of words used to describe something will depend on the context and the desired tone of the writing.