Compare the basic ability between ammonia, ethyl amine and phenyl amine.

In terms of order of increasing basic ability, it is as follows. Phenylamine < ammonia < ethyl amine.
The reason why phenyl amine (containing a benzene ring) is a weaker base than ammonia is because the delocalised ring of electrons in the benzene ring has an electron-withdrawing effect. This results in the lone pair of electrons on the Nitrogen atom of phenyl amine being drawn into the ring. Therefore, the ability of phenyl amine to accept a proton is reduced and so it has less basic ability than ammonia.
On the other hand, ethyl amine (containing an ethyl group), is a stronger base because the ethyl group has an electron-donating effect. This makes it easier for the Nitrogen to accept a proton with its lone pair of electrons and therefore ethyl amine has improved basic ability over ammonia.