Which compound has the least extent of delocalisation of the nitrogen lone pair into the benzene ring?

The $\mathrm{N}(\mathrm{CH}{}_3){}_2$ group can conjugate with the ring only if it is coplanar with it. The two ortho methyl groups crowd the N-methyl groups and force the nitrogen out of the ring plane, so the lone pair overlaps poorly with the pi system (steric inhibition of resonance).