The phrase "neck and neck" refers to a close competition or race between two equally matched opponents. Antonyms for "neck and neck" would therefore refer to a situation in which one opponent has a clear advantage over the other, whether through skill, speed, or other means. Some possible antonyms might include "one-sided," "lopsided," "dominant," "uneven," "lopsided," or "unequal." These words suggest a situation in which one party is likely to come out on top, rather than a situation in which the outcome is uncertain or could go either way.