I agree with Selmo - and with your point, Thinlómien, that we could hardly define ranks of some sort in this way. How would we compare, let's say, a minister of interior and an army general? That's similar, I think. About the titles, we also know that he was the (help me find the word - I don't know how it is in original) Lieutenant (??? insert correct word, please) of Barad-dur.
In what I don't agree with you, Selmo, is that MoS was not "powerful" in the "supernatural" sense. It is written that he
learned great sorcery, which would probably mean the most powerful sorcerer besides Witch-King (or at least most powerful from the Men in this case). Not that he could stand up against Gandalf (certainly not after his rebirth), but then again, who knows? I can as well imagine a scene of Gandalf falling into the Pit of Khazad-dum after a battle with the Mouth instead of the Balrog. Well, that's maybe too much. But it would be nice, don't you think
And one last think, I like the idea of him being the one sent to Erebor (and Dale), I find it quite possible and I also thought about this possibility before. Although it is not for certain and I'm suspicious that all of Sauron's messengers look the same (some black guy on a horse), so that might have been anyone
