The word "unstablest" refers to something that is the least stable or the most unstable. The antonyms for this word include stable, fixed, steady, unchangeable, constant, balanced and secure. Stable means firm, steady and not easily moved or changed. Fixed means attached or placed in a permanent position. Steady refers to something that is unvarying or consistent. Unchangeable means something that remains the same despite any external factors. Constant refers to something that is continuous or unending. Balanced means something that is equal or evenly distributed. Finally, secure means something that is safe and protected from danger or harm.