Aniline is a weaker base than aliphatic amines (like methylamine) due to the Resonance Effect.

• In aniline ($C_6{H}_{5}N{H}_2$), the lone pair of electrons on the nitrogen atom is in conjugation with the $\pi$-electron system of the benzene ring.
• Through resonance, the electron density is delocalized over the ring, making the lone pair less available for coordination with a proton ($H^{+}$).
• Furthermore, the anilinium ion formed after protonation has fewer resonance structures than aniline itself, making it comparatively unstable.
• Aliphatic amines have no such resonance and instead benefit from the $+I$ effect of the alkyl group.