All right, got more free time than I thought today, so here I go. I probably could organise this more, but I feel like the argument will continues after this anyway.
Nigh-Valar tier/Chief Maiar tier - What is the foundation
The Silmarillion has this line "And the Valar drew unto them many companions, some less, some well nigh as great as themselves, and they laboured together in the ordering of the Earth and the curbing of its tumults." - These "companions" logically refers to fellow Ainur, aka the Maiar.
The Oxford Dictionary says this about the word "nigh"
adverb
near in space, time, or relation: The time draws nigh.
nearly; almost; (often followed by on or onto):
Now it's unlikely "nigh" refers to a spatial relation, as the term less is used to refer to other members of the Maiar as well, implying they are speaking of greatness.
The term "great" too has many meanings, but as I covered in page 4, the language around the Valar is pretty cut and dry. For instance, "The Great among these spirits the Elves name the Valar, the Powers of Arda" - refers to greatness as in might or grandeur, and while this sentence alone does not state this, reading the rest of the chapter, and the Silmarillion as a whole, sees the term "
great" often used in relation to highlight the grandeur, power, and majesty of a thing. For instance, "great fires", "
Great Ones" (in relation to the Valar), "
greater and lesser spirits", etc. Whilst they're also used for other purposes, I'm being overly pedantic here because the point I'm trying to make is that the phrase
"
nigh as great..." - is very cut and dry, and that no nuance needs to be added to the term. This isn't a Literature Exam where we analyse the spectacled billboards.
Non-Canon
Now, this will be a tangent, as whilst I am about to discuss non-canon material, the point I am trying to highlight is intent, which is carried over.
Morgoth's Ring is a source of much abandoned material (alongside some used ones), and tells us much about the direction Tolkien was potentially planning to take with his characters.
For instance, with
Eonwe, we learn that his original conception as Fionwe, son of Manwe and Varda, is rather comparable with the Valar
"
Then shall the last battle be gathered on the fields of Valinor. In that day Tulkas shall strive with Melko, and on his right shall stand Fionwe and on his left Turin Turambar, son of Hurin, Conqueror of Fate, coming from the halls of Mandos...
Moreover, we even have a line in "The Book of Lost Tales" where Eonwe is even the one to kill Morgoth to avenge Arien (with different names naturally)
Osse on the other hand was not just depicted on a comparable, but to also outright be a Vala in his own right in previous conceptions
"
These are the nine chieftains of the Valar that dwelt in Arda: Manwe, Ulmo, Aule, Orome, Tulkas, Osse, Mandos, Lorien,(2) and Melkor"
And one later revision has this to say "
'he [Osse] was not a Vala, but a chief of the Maiar, servant of Ulmo.'"
Now none of this is relevant in and of itself, but will be noteworthy later.
Chief Maiar - Contradictions
Now, I have to ask the question, do the Chief Maiar have any reason not to be Nigh-Valar tier? Do they have glaring contradictions?
Eonwe
Silmarillion/UT - "
One of the mightiest of the Maiar"
Silmarillion - "
Chief among the Maiar of Valinor..."
UT - "
To the overthrow of Morgoth he sent his Herald Eönwë..."
HOME - "
In the Great Battle when at last Eonwe herald of Manwe over- threw Morgoth and Thangorodrim was broken"
HOME - "
Fionwe son of Manwe was the leader and commanding authority in the final war against Morgoth and his overthrow; but the abandonment of that conception, and the change in the 'status' of Fionwe / Eonwe to that of Manwe's herald led to doubt whether my father, had he ever returned to a real retelling of the story of the end of the Elder Days (see XI.245-7), would have retained Eonwe in so mighty and elemental a role. His part was in consequence somewhat diminished by omissions and ambiguous wording (as may be seen by comparing the text in Vol.V with that of the published Silmarillion; cf. also the editorial addition made to the Valaquenta, X.203). There is however no evidence for this supposition, and I now believe it to have been a mistaken treatment of the original text, and so also here in the Akallabeth."
All of the above make it clear that Eönwë is chief among the Maiar, personally overthrowing Morgoth (albeit a weakened one, but still superior to Ancalagon), and leading the defeat of Angband.
However, there is the matter of Ancalagon. While I'm not about to restart the conversation of whether or not he was repulsed by the Winged Dragons (we've both made or stances clear, and there is no budging there), we can prove it to be PIS or an Outlier if he was repulsed.
Eönwë is chief among the Maiar, one of the mightiest of their number, and specifically Herald to Manwë. Thorondor is the greatest eagle of Manwë. Yet we learn of these lines in Morgoth's Ring -
"
Manwe however sent Maia spirits in Eagle form to dwell near Thangorodrim"
"
that the spirits flying from Manwë's halls in the shape of hawks and eagles were borne by their wings 'through the three regions of the firmament"
and whilst this concept was abandoned for some time
in 1970, it reappears for good
"
The most notable were those Maiar who took the form of the mighty speaking eagles that we hear of in the legends of the war of the Ñoldor against Melkor, and who remained in the West of Middle-earth until the fall of Sauron and the Dominion of Men, after which they are not heard of again. Their intervention in the story of Maelor, in the duel of Fingolfin and Melkor, in the rescue of Beren and Lúthien is well known..."
Now we know Eönwë is chief among the Maiar, and it logically stands that he should be superior to Thorondor, who personally assisted in the defeat of Ancalagon the Black. This, combined with the fact he defeats Morgoth, and is likely superior to a Sauron above Ancalagon, makes it clear Eönwë is not a contradiction to the notion of a Nigh-Valar Maia.
Ossë
He shows up so little that he literally cannot be a contradiction. In fact, the only major things we learn from him (that are relevant to this conversation) is that Morgoth desired to use him as a tool against Ulmo
"
Melkor hated the Sea, for he could not subdue it. It is said that in the making of Arda he endeavoured to draw Ossë to his allegiance, promising to him all the realm and power of Ulmo, if he would serve him. So it was that long ago there arose great tumults in the sea that wrought ruin to the lands. But Uinen, at the prayer of Aulë, restrained Ossë and brought him before Ulmo; and he was pardoned and returned to his allegiance, to which he has remained faithful. "
Now, this certainly could refer to Ossë being a powerful tool against Ulmo's domain, and I'd in fact agree, as Ulmo is a member of the Aratar. What is significant is the size of his domain. He is master of all coastal seas, while his counterpart Uinen spreads her hair across all of Arda's surface waters. Such domains make up an impressive chunk of Ulmo's own domain (as Lord of all Waters), and it's implied Ossë is limiting himself - "
He does not go in the deeps, but loves the coasts and the isles, and rejoices in the winds of Manwë; for in storm he delights, and laughs amid the roaring of the waves..." - The quote seemingly hints that he limits his domain to the regions he enjoys, otherwise he may venture into the deeper seas.
Now, this does not mean anything significant for or against my argument on its own, but it does when you consider their origin and the themes. Ossë is set up as a domain-holding vassal of Ulmo (a remarkable thing for a Maia, as even Eönwë is a Herald), and his power is shown able to oppose his domain. This is reflective of the older texts were he is a rival Vala to Ulmo, and capable of contesting his control over the waters. Of course this doesn't mean they're on par, but it does thematically imply Ossë is akin to a Vala in influence (and of course, we know Tolkien considers the two as chief among the Maiar)
Arien
Again, there's nothing to contradict her being at this level. We only have quotes of a weakened Morgoth fearing her, which puts her above Ancalagon automatically, which means no issues.
Melian
Da Capo
Isn't it a bit too much of a difference between tiers?
This is just a classic argument from incredulity. The Valar are the same race as the Maiar, and yet they are astronomically superior to most of the latter.
Isn't "Likely Higher" a better option?
Initially I was heading towards this option, yet this only works if we destroy the initial premise, "...nigh as great as themselves..." If we accept this statement as true, then the Maiar of this level are possibly/likely 3-A, as they are most likely comparable.
If the statement is rejected, there is not point in saying "likely higher" as not gap is present that suggests such an approach
If you say, we don't know if this implies they're 3-B or 3-C, then this is just a bad faith argument, as those tiers are not comparable with 3-A.
Sauron as an issue
Let me just repeat myself here.
To be frank
Sauron being chief among the Maiar is supported only by these things.
He's the mightiest Fallen Maia, yet even Maiar like the Balrog are considered mighty among the Fallen Maiar, despite being outclassed by many of their Valinorian counterparts.
He might scale to Eönwë, but this is not something which is confirmed, and relies on a contradictory extrapolation.
He is considered a greater order Maia to the Istari, but this does not mean he is among the highest of the rank.
Aren't your individual Nigh-Valar tiers flimsy?
To be frank, minus Eönwë, Ossë, Uinen, and Ilmarë, all of the others are up for debate.
But as long as the initial statement remains, the chief among the Maiar are likely comparable to the Valar.