Which is the weakest base in aqueous solution among aniline, 4-methylaniline, 3-methylaniline and 2-nitroaniline?

The nitro group is strongly electron-withdrawing and, in the ortho position, also hinders solvation and protonation of the amino group. The conjugate acid of 2-nitroaniline has $\mathrm{p}{K}_a\approx -0.3$ against 4.6 for anilinium, so it is far weaker than aniline. The methyl-substituted anilines are not weaker than aniline in this series.