The fairest?

[Raynor]

Among the Children of Eru, Luthien sits at the top of the list:

Originally Posted by Of Beren and Luthien, Silmarillion

Then all memory of his pain departed from him, and he fell into an enchantment; for Luthien was the most beautiful of all the Children of Iluvatar.

As for Varda, we know that:

Originally Posted by Valaquenta, Silmarillion

Too great is her beauty to be declared in the words of Men or of Elves; for the light of Iluvatar lives still in her face.

Given the status of the valar, I would personally go with Varda.