a one

/ˌeɪ ˈwʌn/
adjective
  1. Of the highest quality; excellent or first-rate.
    • This is an a-one example of classic architecture.
    • She is an a-one student who always gets top grades.
    • The restaurant received an a-one rating from the food critic.
What does "a one" mean? | whatsthatwordmean | whatsthatwordmean