Mightiest of the Maiar?

[Alcuin]

You elided the quotation from Letters 183. The phrase you have highlighted is actually a footnote to this sentence,

But [Sauron] went further than human tyrants in pride and the lust for domination, being in origin an immortal (angelic) spirit.*
…
* Of the same kind as Gandalf and Saruman, but of a far higher order.

Tolkien indicates in several places in his writings that there are greater and lesser Maiar, just as some Valar were greater than others. And in Manwë’s Council of the Valar for determining how to respond to Sauron’s rising might in the Third Age, when Manwë commands Olórin [Gandalf] to accompany Curumo and the other Maiar as emissaries to Middle-earth to help Elves, Men and Dwarves resist Sauron, Olórin initially objects that he is afraid of Sauron.

(That is all I can contribute to the forum, and only because it is Christmas and so vacation. Merry Christmas to The Barrow Downs.)