Where does this quote come from?

[The Might]

The Silmarillion provides quotes that should answer your question:

Luthien is of course the fairest:

And of the love of Thingol and Melian there came into the world the fairest of all the Children of Ilúvatar that was or shall ever be.

Beren seems to think so to :

For Lúthien your daughter is the fairest of all the Children of the World.

Galadriel is the fairest amongst the Elves who remained:

A queen she was of the woodland Elves, the wife of Celeborn of Doriath, yet she herself was of the Noldor and remembered the Day before days in Valinor, and she was the mightiest and fairest of all the Elves that remained in Middle-earth.