The correct order of nucleophilicity is

Nucleophilicity  is directly proportional to the electro density on donor atom and the size of donor atom (in gas)

And Stronger the base, greater the nucleophilicity.

As weak acid will have a strong conjugate base ${\mathrm{NH}}_2^{-}$is stronger base than ammonia.Hence, ${\mathrm{NH}}_2^{-}$ is a stronger nucleophile than ammonia