Before JKR outted DD, he and McGonagall were my top suspects.
In Deathly Hallows, right before the Battle of Hogwarts, JKR writes that a "significant look" was passed between McGonagall and (I think, I don't have my book handy) Sprout. I remember the first time that I read it I wondered if there was something more than met the eye to that line.
We'll probably never know for sure, and it's not as though it even matters-but it makes for interesting speculation.