Least human...
Well exempting all summoning animals, who are clearly not human, and the Bijuu, who may not be human, but seem to have more humanity than some humans...
Can I vote for Raza, the 4th Kazekage? I mean, how inhuman do you have to be to put a demon in your unborn son, convince his dead wife's brother to lie to the child and say that he was never loved, declare the child a failed experiment at the age of six, then make multiple attempts on his life, before dying after ordering said son to destroy an allied village. He saw his own son as a monster and a weapon, then was brought back to life only to be astonished at the fact that his son actually has friends.
I say that Gaara's father takes the prize. The guy didn't even change his ways until he died a second time.