The basicity of amines depends on the availability of the lone pair of electrons on the nitrogen atom for protonation.

1. Aniline: The lone pair of electrons on the nitrogen atom is delocalised over the benzene ring through resonance, making it less available for protonation. Thus, aniline is a weak base.
2. Diphenylamine & Triphenylamine: The lone pair is delocalised over two and three benzene rings, respectively. This extensive delocalisation makes them even less basic than aniline.
3. p-Nitroaniline: The nitro group ($-NO_2$) is a strong electron-withdrawing group (-I and -R effects). It further decreases the electron density on the nitrogen atom, making it less basic than aniline.
4. Benzylamine ($C_6H_5CH_2NH_2$): The nitrogen atom is attached to an $sp^3$ hybridised carbon atom, not directly to the benzene ring. Therefore, the lone pair of electrons is localised and not involved in resonance with the ring. This makes the lone pair readily available for protonation. Consequently, benzylamine is more basic than aniline.