Most first-string is an expression that depicts the highest level of proficiency or priority in a particular area. On the other hand, some antonyms for most first-string could be phrases like least proficient or least prioritized. Other examples of antonyms for most first-string include second-string, substitute, or benchwarmer, all of which signify non-primary or lesser importance. In some contexts, the terms intermediate or average may also serve as antonyms for most first-string. The precise antonyms for this phrase depend on the specific context in which it is used.