No, he does seem the most human. Carbuncle, Bismarck, and the like are too bestial. Phantom is too generic. Efreet couldn't have been the father unless Shiva was the mother, since those two are always paired. Crusader, Lahmu (Ramuh), and so on already have their little subplots.

Besides, I don't think they made all the Phantom Beasts and then chose the father out of the lineup. Rather, I think they made Madin as a character and then decided to make it possible to acquire his Magicite.