A one

[ ey wuhn ]
/ ˈeɪ ˈwʌn /

adjective

noting a vessel regarded by a shipping-classification society as being equipped to the highest standard and with equipment maintained in first-class condition.
Also A number one. Informal. first-class; excellent; superior: The meals there are A one.
Also A-one, A 1, A-1.

Origin of A one

First recorded in 1830–40