Galadriel VS The witch- king

[Boromir88]

The quote is everywhere and I'm surprised that you haven't heard of it. From the smallest sites to the Encyclopedia of Arda, it's quoted that Sauron is the greatest everywhere.

Actually I can provide quotes to show that Sauron was not the mightiest of the Maiar:

Chief among the Maiar of Valinor whose names are remembered in the histories of the Elder Days are Ilmare, the handmaid of Varda, and Eonwe, the banner-bearer and herald of Manwe, whose might in arms is surpassed by none in Arda. But of all the Maiar Osse and Uinen are the best known to the Children of Iluvatar~Valaquenta: Of the Maiar

Osse and Melian could also be powerful candidates:

Melkor hated the Sea, for he could not subdue it. It is said that in the making of Arda he endeavoured to draw Osse to his allegiance promising to him all the realm and power of Ulmo, if he would serve him.~ibid

Melkor was unable to control the sea, so he tries to sway Osse to his side to wreak havoc.

All that is ever said is that "Sauron was the greatest of Melkor's servants" (Valaquenta and The Index). Where we are told that Eonwe was one of the greatest among the Maiar (The Index), so Eonwe certainly seems more powerful than Sauron. Osse and Melian there's nothing so straight forward and clear. Melian was used as a shield against Morgoth, which makes me think she was one of the mightiest. And Morgoth, in order to control the sea tries to persuade Osse to join him.