Mightiest of the Maiar?

[Belegorn]

Originally Posted by Zigûr

Melkor being originally most powerful of the Ainur despite Manwë being the leader.

Not only that. In the beginning he was stronger than all their combined strength, "so great was the power of his uprising that in ages forgotten he contended with Manwë and all the Valar" [Sil, p. 26].

Sauron himself was known as Melkor's mightiest servant, not the mightiest of the Maiar. I'm not sure we can gauge the power of most of the Maiar since few are known even to the Elves for they tended to remain hidden, that is, unclad. Some were almost as great as the Valar as is said, so there were quite powerful ones. Of the chief ones known, Manwë's banner-bearer is said to be "surpassed by none in Arda" [p. 24] in might of arms.

Also with Melkor it seems with each succeeding capture he is taken captive by lesser and lesser spirits. First it was against the Valar and then Tulkas took him in. Then at the end of the First Age he was taken in by a Maia, Eönwë! This further shows how much his power was spent when in the beginning he could go against all the Valar on his own.