The term "second-string" refers to something or someone who is of lower importance, quality, or ranking. Antonyms for "second-string" include primary, top-notch, leading, principal, and top-tier. These suggest the highest level of importance or quality. Other antonyms for "second-string" are essential, crucial, necessary, vital, and indispensable, which indicate something that is indispensable or critical. Additionally, important, significant, and prominent are the antonyms for "second-string" which mean noteworthy or high-ranking. Overall, using these antonyms gives a more positive connotation and reflects something or someone of high value or quality.