Among 2-nitroaniline, 3-nitroaniline and 4-nitroaniline, the strongest base is

From the meta position the nitro group withdraws electrons only by the -I effect. From the para (and ortho) position it also withdraws by the -R effect, delocalising the nitrogen lone pair onto the nitro group, and the ortho isomer additionally shows the ortho effect. The pKa values of the conjugate acids are about 2.5 (meta), 1.0 (para) and 0.3 (ortho).