The phrase "a notch under" implies a slightly lower ranking or level than something else. Its antonyms would be "a notch above" or "a step ahead." These phrases indicate a higher level or ranking than something else, suggesting that the subject is superior or has an advantage. Other antonyms might include "equal to," "on par with," or "at the same level as," indicating that the subject is on the same level as something else, with no superiority or inferiority implied. Depending on the context, other antonyms might be appropriate, but these examples give a basic idea of the range of opposites for "a notch under.