The Maiar (individuals)
The hierarchy of the Maiar is left a lot more vague than that of the Valar (which itself is far from detailed), and indeed, descriptions inform as that their might could vary widely from essentially low level spirits, to living elemental entities and angelic spirits.
So instead, I'll try and outline the basic ideas of where the individual Maia stand in relation to each other, and let people (please) debate the minutiae.
The "Chief" Maiar
Tolkien outlines that amongst the Ainur servants who followed the Valar, there were "some less, some well nigh as great as themselves...". (From the Silmarillion,) This rather evidently points to the existence of members of the Maiar who were likely comparable in might to the Valar themselves.
We are not given details as to who may be considered to count amongst this number, but what we do know is that among the Maiar, there were 2 who were considered "chief" amongst their ranks.
They were Eönwë and Ilmarë (accounted in Morgoth's Ring, and the Silmarillion)
Of course, a simple statement like that is somewhat lacking, so it is quite fortunate that such is well evidenced in the case of Eönwë, as he has been accounted to have been amongst the most martially apt of the Maiar. Indeed, his description notes him to have been
"banner-bearer and herald of Manwë, whose might in arms is surpassed by none in Arda" (Silmarillion)
Thus placing him at the top of the totem of martial prowess for the Ainur of Arda, albeit not necessarily power. What does put him at the top of the totem of power for the Maiar, is the fact that the Unfinished Tales makes it quite clear that it was his might alongside his skill, that made him ideal for the "War of Wrath" against Morgoth
"To the overthrow of Morgoth he [Manwë] sent his Herald Eönwë. To the defeat of Sauron would he send some lesser (but mighty) spirit of the angelic people..."
The emphasis on lesser yet still "mighty", is quite evident for how powerful Eönwë was among the Maiar.
Moreover, there's the fact that if you take the above quote into account, and the fact the Silmarillion attests Eönwë removed the Silmarils from Morgoth's crown after he was bowed (after being a coward and begging for mercy, but even a coward would defend himself after assault), and the account from the "Peoples of Middle Earth" of Christopher's regret of removing Eönwë's personal defeat of Morgoth, it can be extrapolated that Eönwë personally defeated the weakened Morgoth, with seemingly little effort.
Of course, Morgoth was far reduced by this point (being harmed by the likes of Fingolfin) and was certainly not in the correct mind-set to resist with what was left of his might (begging for mercy does that), but the seeming ease with which Eönwë defeats him, only makes it clearer that he was certainly one of the chief powers of the Maiar, given Morgoth was likely still mightier than his servants at this time (with the possible exception of Sauron...)
Moreover, since Ilmarë is put in the same regard as Eönwë, she should likely be comparable, if not somewhat inferior, due to her lack of emphasis on martial affairs.
To conclude on this part, I believe it's safe to say these two were among the Maiar said to be peers of the Valar in might, and that they scale to at least the .
(And of course there's the earlier versions were Eönwë was the son of Manwë, and one of the possible candidates to slay Morgoth at Dagor Dagorath, but that's neither relevant, or canon).
The Sun and Moon - First Age Edition
Arien and Tilion were the Maiar chosen to guide the vessels of the Sun and Moon respectively (though the vessels and their shape are contested).
Of the two, Tilion notably appears weaker
For instance, the Silmarillion and Morgoth's Ring notes that the "flame of Anar [the Sun] scorched him", though of course, that could be the result of a difference in chosen form, as Arien was a "spirit of fire", while Tilion was a hunter of Oromë.
What's more poignant is Morgoth's response to their existence, as for Tilion, the Silmarillion notes that he
"assailed Tilion, sending spirits of shadow against him... but Tilion was victorious"
after an extended conflict mind you. In contrast, to Arien
"Morgoth feared with a great fear, but dared not come nigh her, having no longer the power..."
Now mind you, this could certainly mean that Morgoth was weaker than Arien (which would certainly justify Eönwë's single-handed defeat of an even weaker version of him), but it could also mean that Morgoth literally lacked the power, having lost his ability to enter a bodiless form, as noted in a prior segment from NOME, and being implied in the next line, following his wasting of more power in corrupting the world and its inhabitants to fear the Sun...
"... his might passed into the and was dispersed, and he himself became ever more bound to the earth."
Regardless, the point becomes evident that Morgoth held servants whom he believed had enough might to attack Tilion, but not Arien, evidently showing a difference in might.
This puts Arien above Tilion, and potentially above a First Age Sauron, and Weakened Morgoth, potentially putting her into the nigh-Valar tier? This will need more input than just me though.
Tilion is simply fairly mighty among the Maiar, being capable of defeating seemingly powerful spirits of Morgoth (given he feared the Moon too, he would have spared some mightier servants), but nothing too special, he wouldn't have a particular place in the scaling chain.
The Queen of Doriath
Melian is another fairly impressive member of the Maiar, being held as both mighty amongst the Maiar, whilst holding feats to prove it.
For instance, when the Valar elected to send several Maiar guardians to guide the Elves to Valinor (at their free will) 5 were elected, with Melian as a 6th, their leader. Of these Maiar, it is said that they were the same individuals who would become the Istari, and just as their might was emphasised earlier from the extract from the Unfinished Tales, in NOME, it is said that the number was made up of
"...five Guardians (great spirits of the Maiar) - with Melian (the only women, but the chief)
The emphasis on Melian as chief among the Maiar may perhaps only refer to rank, but it is possible/likely it refers to power, and regardless, she almost undoubtedly scales to them in power, considering the Valar specifically sent peers who could be considered as "great" to guide and protect the Elves from the forces of Morgoth, which were considerable, with Sauron and the Balrogs yet being active.
Beyond this meeting, Melian doesn't get too many statements of her power, but we do have one more major feat - her repulsing of Ungoliant.
Now then, do not mistake this as putting Melian above Ungoliant, as such would completely break scaling, given Ungoliant had just subdued a Morgoth who was still regarded as "... the mightiest of all the dwellers in Eä" (from "of the Silmarils" in the Silmarillion), which obviously includes Melian. One needs to look at the context.
"... Ungoliant fled from the north and came into the realm of King Thingol... by the power of Melian she was stayed..."
Now such is indeed a feat, but now as great of one as one may initial think. To be frank, Ungoliant acts as an outright animalistic entity, and just as an animal, she appears to quite quickly flee from any sign of danger, likely as her great hunger (which the Silmarillion notes, was so great it not only lead her to eat her children, but possibly lead to her consuming herself) makes her fear losing any energy.
Such is admittedly speculation, but I believe it's a well-supported one, as when faced by the Balrogs, entities far weaker, if barely comparable to her (if even that), she fled
"Balrogs.... with their whips of flame they smote asunder the webs of Ungoliant, and she quailed, and turned to flight..."
Such is quite clear evidence of her cowardice, as in the face of even a modicum of stern resistance (note the text does not clarify if she was even injured), she flees.
That being said, such was a feat that took all the Balrogs to replicate, and while it's possible it could have taken less, it cannot be denied that Melian replicated a feat that, from what we know, took all the Balrogs to accomplish.
Overall, she is clearly considered mighty among the Maiar, being a peer or possible superior of the Istari, of whom the number includes Gandalf, whom even restricted to the body of an old man, could defeat a Balrog (with his power unveiled yes, but still in an old man's body), while she herself demonstrates a feat that may put her on par with the combined might of the Balrogs, although this would be a high-end.
Ossë and Uinen
Ossë and Uinen are not given much statement in regards to the placement in the hierarchy of the Maiar, but from the evidence given, we may extrapolate they were fairly mighty.
Ossë is regarded as a direct vassal of Ulmo, which is rather significant, as such places him above the mere servile roles most Maiar seem to adopt, as messengers or maids.
What's more useful, is the fact that he was regarded as
"the master of the seas that wash the shores of Middle-earth"
which appears to be a voluntary limitation, as it's mentioned in the immediate following that he
"He does not go in the deeps, but loves the coasts and the isles..."
Now all this sounds like fluff, and it kinda is, but it's useful fluff, as it's then stated in the Silmarillion's detailing of Ossë, that Morgoth
"....hated the Sea, for he could not subdue it..." (Silmarillion)
Thus implying Ossë's might over the waters surpassed Morgoth's own, which is rather impressive, and automatically places him in the upper echelons of the Maiar.
Indeed, Ossë's might was rather impressive, to the extent Morgoth desired to recruit him according to the Silmarillion, and he temporarily does.
"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... So it was that long ago there arose great tumults that wrought ruin to the lands"
It was not until his wife Uinen calmed him, that he repented and served Ulmo again. Speaking of Uinen, we have even less to go off of her, but what we do know is that she had a remarkable levels of authority over the waters of Arda, given she quite literally existed in all the waters of the surface
"Uinen, the Lady of the Seas, whose hair lies spread through all waters under sky..."
Such alone implies a high standing among the Maiar, given that few have such high and lofty positions of authority as to claim being a "Lady of the Seas".
However, it must be said that, while we can extrapolate Ossë as being of the Maiar comparable to the Valar from Morgoth's regard for him, and his own inability to subdue him, it is quite telling such is only in one regard, and that Ossë lacks any real statement of his being mighty among the Maiar.
Of course there is legacy stuff, such as in the early versions of the Silmarillion, where he was portrayed as a Vala (alongside Uinen) somewhat opposed to Ulmo, but such is no longer canon.
Honestly, I personally view him as potentially being comparable to the Valar, but I have my doubts. Uinen is more cut and dry, as her high position, merely implies a degree of might among the Maiar, and she doesn't have much special directly going for her scaling wise beyond that.
To sum up
There are more members of the Maiar to cover, the Istari, Sauron and the Balrogs to say the least, but they are unique enough to deserve their own sections.
To sum up my personal thoughts on scaling:
Maiar comparable to the Valar should receive the flat High 3A, likely Low 2C ranking for their "pre-Eä" key, and a similar comparability again for what is agreed for their "in-Eä" key (which I believe should be At Least 3B, given individual Valar had great control over the material universe)
These Maiar would outright include:
Eönwë
Ilmarë
Possible inclusions may be:
Arien
Ossë
(Melian is iffy for reasons above)
Regarding the rest of the scaling, I apologise, but it really is rather vague. All we know is, they are fairly strong for the Maiar... Of whom we do not have enough information to make an average for, outside of pre-Eä.
What we can say is that:
The Istari (without their limitations) are considered to be mighty among the Maiar. Melian is a peer of them, and a possible superior.
Arien is a powerful Maia, whom even a diminished Morgoth seems to fear, and she is seemingly mightier than Tilion, who is undoubtedly mighty himself, but vaguely so, as besides being a martial Maia, due to being a hunter, little special emphasis is put on his might.
Ossë, without his one possible nigh-Valar tier feat, is given a lot of high authority for a Maia, but is completely unknown in the hierarchy, and the most we can say is that the Istari may be peers or superiors of him (only if the nigh-Valar feat is not considered), due to the lack of the text's emphasis on his might. But this is so flimsy.
We still have the Istari, the Balrogs and Sauron himself to cover, so changes to this may come later, in that.
