Top and even Mid lane, that way he can farm up his Q and be a monster tank+do heavy damage at end. People scoff at him midlane, but he's awesome there, all you have to do is survive early level pokes, which isn't hard, and continue to farm up, even if it's under the tower. After a certain level not many champs hurt you much if your build is right, with exception of teemo, because teemos damage needs a nerf. Also the only really hard counter i've seen for nasus in a 1v1 situation is probably quinn, her kickback is unstoppable+it slows you+lets her unload so it's a rough day against a quinn.