1.0 The Ranking System
The main aim of this article is to focus on the order of rankings starting from Barad-dûr and proceeding to Minas Morgul. There is no question though that 
at the very top of everything, there is Sauron (after all, he’s the Lord of the Rings!).
He is the Dark Lord of his realm. As a start, there is no difficulty in finding who had command over all the land and its organisation.
Trouble starts when trying to establish (in a specific order) who was under Sauron himself, and had much power and control over the realm. One can well start mentioning characters who could have easily been “second-in-command” in Mordor, such as the Nazgûl and the Mouth of Sauron.
At the beginning of this article, I mentioned that there are a number of quotes in The Lord of the Rings that immediately establish the idea that a kind of command system really did exist.
“’Nazgûl, Nazgûl’, said Grishnakh … ‘you ought to know that they’re the apple of the Great Eye.’ ”
– The Lord of the Rings: The Two Towers, Book 3, Chapter III
Also, much of the following speculation will be based on the piece of the dialogue between the orcs, Gorbag and Shagrat from The Two Towers:
Gorbag – “ ‘They don’t tell us all they know do they? Not by half. But they can make mistakes, even the Top Ones can.’ “
Gorbag on the Nazgûl: “ ‘But He likes ‘em; They’re His favourites nowadays … it’s no game serving down in the city.’ “
Shagrat on Gollum: “ ‘ He’s been here before. Came out of Lugbûrz the first time, years ago, and we had word from High Up to let him pass.”
– The Lord of the Rings: The Two Towers, Book 4, Chapter X
The words “Top Ones”, “His favourites”, and “High Up” are but some of the many words indicating an authoritarian system. Understandably, the Nazgûl seem to have attained much power, although if we go back to Gorbag’s remark : “ ‘But He likes ‘em; They’re His favourites nowadays’”, we notice the word “nowadays”.
This can conjure up all sorts of questions: “Who were Sauron’s favourites before?”, “Why is it so nowadays”, “What did the Nazgûl do to achieve such preference?”.
Though, on this matter, we shall return later.
1.1 Barad-dûr or Minas Morgul?
There is no question that when coming to commanding and issuing orders, Barad-dûr is at the top of the list, and it is only evident that it’s the main tower and the ‘home’ of the Dark Lord himself. But its accompanying structure, Minas Morgul, seems also to be a centre-piece in Mordor’s events. There was certainly a type of command chain between both towers, as seen from this quote:
“ ‘owing to the Great Signal going up, and the High Nazgúl going off to the war.’”
– The Lord of the Rings: The Two Towers, Book 4, Chapter X
The “Great Signal” was issued from Barad-dûr, signalling Minas Morgul to send its army against Gondor; before Frodo, Sam and Gollum proceeded to the Stairs of Cirith Ungol. The “High Nazgúl”, most certainly is the Witch-King himself.
This leads us to discuss the title in question: if the “Top Ones” and orders from “Higher Up” are referring to the Nazgûl, then under which tower was their authority present?
Under Barad-dûr (seat of their Lord) or Minas Morgul (their own city)?
It would be tempting to assume both. To explain this matter in simpler terms, let us take the Witch-King himself.
Throughout The Lord of the Rings he is given many names, one of which is the “Black Captain”. Now, looking at the meaning of the word “captain”, one can explain it as being: someone who holds a rank and is above a lieutenant.
1.2 The Mouth of Sauron
For now, keeping the above in mind, we will put aside the characters of the Nazgûl and shall discuss a second important figure – the Mouth of Sauron.
He is described as being the Lieutenant of the Tower of Barad-dûr. As I noted above, a Captain (the Witch King) has a higher rank than a lieutenant (the Mouth of Sauron). That is why the classification of the Nazgúl under one of the Towers is important. It is not made clear in The Lord of the Rings if the “Top ones” and “Big Bosses” are referring to the Nazgûl’s command in Barad-dûr.
So, let us assume again that the Nazgûl had high rankings in the Dark Tower, whilst also commanding at Minas Morgul.
(In a present day situation, Minas Morgul can be seen as the ‘home’ of the Ringwraiths, whilst Barad-dûr is their ‘work place’).
It would thus be tempting to lay out a diagram indicating (in order of importance) the rankings of the Dark Tower, taking into consideration the meanings of both “captain” and “lieutenant”.
The difficulty comes in trying to establish who had the highest rank, and therefore, the highest command and priorities (under Sauron); whether the Lieutenant of the Tower of Barad-dûr or the Nazgûl (more specifically, the Witch King of Angmar).
Highest Ranking Positions in Mordor
I classified the rest of the 8 Ringwraiths separately and not on the same level as the Witch King, due to his title of Lord of Morgul and of the Nazgûl – seriously suggesting the much-obvious among Tolkien fans – that he exerted almost supreme power in Mordor (just under Sauron himself).
1.3 Dates and Physicality
Let us first consider both the Ringwraiths’ and the Mouth of Sauron’s emergence. According to Appendix B in The Lord of the Rings, we can track the first appearance of the Nazgûl 
under the service of Sauron. This dates back to SA2251 and at the time of the War of the Ring, 4208 years had passed.
As to the Mouth of Sauron, “he was a renegade, who came of the race of those that are named the Black Númenóreans” (The Lord of the Rings: The Return of the King, Book 5, Chapter X).
Considering he was under Sauron’s influence (and a Númenórean), his lifepsan would have far exceeded that of his own race – but certainly nowhere close to that of the Ringwraiths.
It would seem evident that the Nazgûl (being in a way immortal) retained their high status and power throughout the thousands of years; and that the title of Lieutenant of Barad-dûr (undoubtedly, through some form of inheritance) would have been occupied by dozens of other corrupt Men – before the one we meet during the War of the Ring.
However, this issue of mortality still doesn’t prove that the title of the Mouth of Sauron was below that of the Ringwraiths.
Being the Dark Lord’s emissary and his own messenger (even, the ‘voice’ of Sauron himself) was undoubtedly a position of privilege and very high power.
***
Tolkien seems to mention the Nazgûl as Sauron’s “most terrible servants”; however, this doesn’t necessarily mean that they were his highest servants.
One could easily think that, since the Ringwraiths, being more or less spirits or wraiths, their physical power was certainly limited and it might be speculated that someone of much greater physical strength (or command) would claim their place directly beneath Sauron’s.
1.4 The Tyrant of the West
In this ongoing debate between the Nazgúl and the Mouth of Sauron, once again we need to examine a peculiar quote that seems to capsize the whole situation. During the parley in front of the Black Gates, the Mouth of Sauron assures his lord’s victory:
“But they [the men of Rohan] shall help to rebuild Isengard which they have wantonly destroyed, and that shall be Sauron’s, and there his lieutenant shall dwell: not Saruman, but one more worthy of trust … He was to be that Lieutenant, and gather all that remained of the west under his sway; he would be their tyrant and they his slaves.”
– The Lord of the Rings: The Return of the King, Book 5, Chapter X
Here is a typical example of the power the Mouth of Sauron exerted in Mordor. If Sauron’s domination was successful, he would have been in control of all the lands to the West and reside in Isengard as lieutenant – an honour certainly given to a high ranking servant.
The question at hand is this: if Sauron had control over Middle-Earth and the West was given to the Mouth of Sauron (with the East falling under the Dark Lord himself), what would the Nazgûl earn for their victory?
When we are told they’re the “greatest servants”, are they literally just servants? Pawns used for sheer fear impact and for controlling the armies on behalf of Sauron himself?
Whatever the answer, the above quote surely shows that the Lieutenant of the Tower of Barad-dûr exerted strong influence and would have been one of the favourites of the Dark Lord – unless his idea of a “Tyrant of the West” was only a deceit created by Sauron himself on his own highest of servants (“Sauron the Deceiver”)…
1.5 A definite truth?
So, who ranks higher? The Witch-King of Angmar or the Lieutenant of Barad-dûr?
It all depends on the way you look at it.
You can either say that Tolkien gave several clues that the Nazgûl were his greatest servants and that among them, the Lord of Morgul was the strongest.
Or else you can favour the Mouth of Sauron as being an alternative option.
***
In Part II of the article, an illustration will present the overall (speculated) rankings of Mordor’s military structure.
2.0 The Military Structure
It is no use denying that Sauron’s domination depended mainly on his army and his ability to keep control of its large numbers by using his captains. Orcs were the Dark Lord’s main forces and it was they who would do most of the work on his behalf.
2.1 Captains/Commanders
From The Lord of the Rings we learn that captains organized the entire army and they were set at various locations around Mordor and in appropriate orc divisions. Two of the most famous captains were Shagrat and Gorbag – the former under Cirith Ungol and the latter under Minas Morgul:
Shagrat: “I’m in command of this pass [of Cirith Ungol]”
‘Then not far inside, or so he [Sam] thought, he heard the two captains’ voices talking again. Perhaps Gorbag, who seemed to belong to Minas Morgul.’
Gorbag: “By all signs, Captain Shagrat, I’d say there’s a large warrior loose …”
– The Lord of the Rings: The Two Towers, Book 4, Chapter X
Both these characters (most probably uruks), control passages that, even though not really essential due to the guard of Shelob the spider, were still given priority by Sauron to be patrolled.
Apart from orcs, there would certainly have been Men who occupied positions as captains and most probably they would have commanded those who came from the East and from Harad. But of this matter, I will soon be discussing it in Part II.
2.1.1 Uruks vs Orcs
It’s a general fact that there exists a difference between the Uruk-Hai and the Orcs. The Uruks are stronger, stouter and immune to sunlight, as opposed to the orcs. Thus it seems that whilst reading The Lord of the Rings, the Uruks are a far stronger breed and therefore occupy higher positions and preferences than the orcs.
They seem to fill-out the majority of roles and oversee important tasks. As Gorbag puts it:
“Always the poor Uruks to put slips right, and small thanks.”
– The Lord of the Rings: The Two Towers, Book 4, Chapter X
Once again, we find another quote in the book that shows such idea:
“They [orcs] were a gang of the smaller breeds being driven unwillingly to their Dark Lord’s wars; … Beside them running up and down the line, went two of the large fierce uruks, cracking lashes and shouting.”
– The Lord of the Rings: The Return of the King, Book 6, Chapter II
This quote gives quite strong evidence of the power that the uruks exerted on the lesser breed of orcs and we can clearly establish that Uruks were higher ranking than the other orc breed.
2.1.2 The Grishnákh Case
Grishnákh is an orc, appearing briefly in the opening chapters of The Two Towers, forming part of the captors taking Pippin and Merry to Saruman.
He is the main conspirator (and leader) of the Mordor Orcs, against Uglúk (leader of the Uruks of Isengard). It is curious to notice Grishnákh’s behaviour in the last pages of Chapter III.
At this point in the story, the Orcs and the Uruk-Hai are surrounded by the Rohirrim and Grishnákh is alone with the two hobbits.
Since this dialogue is long, I have shortened it considerably and have written down only the essential fragments:
“The thought came suddenly into Pippin’s mind, as if caught direct from the urgent thought of his enemy: ‘Grishnákh knows about the Ring! He’s looking for it, while Uglúk is busy: he probably wants it for himself.’”
Merry: ‘Do you want it, or not? And what would you give for it?’
“Do I want it? Do I want it?’ said Grishnákh, as if puzzled; but his arms were trembling. ‘What would I give for it? What do you mean?’”
Merry: ‘Saruman will take all that he can find. If you want anything for yourself, now’s the time to do a deal.’
“Grishnákh began to lose his temper … ‘Have you got it – either of you?’ he snarled.”
So Grishnákh knew about the ring. How? How could a messenger of this sort know of such a powerful object?
“They [those at Lugbúrz] might agree with me; with Grishnakh their trusted messenger.”
– The Lord of the Rings: The Two Towers, Book 3, Chapter III
Even if he was a high-ranking orc, Sauron would certainly not have allowed any knowledge of his Ring. Maybe Grishnákh was a spy of sorts and somehow managed to get information which could have slipped down from higher ranks.
Whatever the reason, mistakes or not, information was not so easily disclosed and such an important passage can only demonstrate that Grishnákh might have held quite a high rank among the military structure of Mordor.
(End of Part I)
