Mightiest of the Maiar?

[Boromir88]

Physically, I think Eönwë may be argued to have the most physical power, as he is said in The Simarillion to be unsurpassed in arms by "any in Arda".~Inzil

Right, and if we are talking physical might I would not put Sauron at the top, because the times when he has gone into combat, he's lost or been killed. Granted he apparently took on Elendil and Gil-galad at the same time, but he was killed nonetheless.

And The Silmarillion says that Sauron is the greatest of Morgoth's servants, but that is an important qualifier.

Tolkien writes Sauron was wiser than Melkor and understood more of the Music as well. So, he has that going for him even though Olorin is counted as the "wisest" of the Maiar.

I think Sauron's greatest might, and I can't think of a better word for it than this, is spiritual. That is he was a master of bulldozing others' free wills and making his servants' wills act in accordance to his will. Corrupting Ar-Pharazon and he pretty much got an entire island destroyed without lifting a weapon. The Ringwraiths were completely enslaved to his will and even with the orcs he managed to organize and control them to a greater extent than Morgoth.