The N which contributes least to the basicity of the compound is:

N-1, N-3, and N-7 are pyridine-like nitrogens. Their lone pairs are localized in $$sp^2$$ orbitals and do not participate in aromaticity, so they are available for donation. N-9 is pyrrole-like (NH). Its lone pair participates in the aromatic $$π$$ electron system of the ring to maintain aromaticity.

The lone pair on N-9 is the only one involved in the aromatic sextet of the five-membered ring. Therefore, it is the least available for donation, making N-9 the least basic nitrogen in the compound.