I am the Mouth of Sauron.

[Legate of Amon Lanc]

Yeah, I think that's it. Except...

Originally Posted by ninja91

Well, I think that he may have a higher rank than the Witch King because he is the lieutenant of Barad-Dur itself. Is that not a rank higher than general of the Morgul-host?

...I still think that this is something else. WK was much as the general of the Morgul-host, but he was also, how to call it, "prefect of Morgul". MoS was something like mayor of the capitol city, but nowhere it's written that he was higher rank than the Lord of Nazgul. After all, Denethor refers to the Witch-King as "the main captain of the host of Barad-dur". It may be metaphoric or his low knowledge (which I doubt, since he was referring to what he knew possibly verified by what he saw in Palantír), but this would seem that if there were any "ranks", WK is at the top. Which would mean, the Mouth is lower.
...Okay, I confess, I am racist. I think MoS was human so that he could not reach such a high rank as any Ringwraith.
But actually, I think MoS was more active in the "civil" section and WK had military rank, thus, you cannot compare them. So WK was the highest in the army ranks and MoS was... something else.