First-class and One-off

First-class adjective - Of the very best kind.

One-off and first-class are semantically related. In some cases you can use "One-off" instead an adjective phrase "First-class".

One-off adjective - Being the one or ones of a class with no other members.

First-class and one-off are semantically related. Sometimes you can use "First-class" instead an adjective phrase "One-off".