Well, if you want to get technical, the first true Avatars, in Hindu Mythology, were actually incarnate manifestations of the gods. And since it is unknown if the people in Aang and Korra's world worshiped gods, the spirits are the closest thing. And since Aang and Korra are manifestations of a connection between the mortal world and the spirit world, that would make
them the closest non-divine equivalent of that. Meaning that I'm right. Aside from literal manifestations of gods, Aang and Korra, and the line they reincarnate from would come first.
Just sayin'.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)