As you say, the best is something that has never really existed. It only came into being when one of the little boys talked about it and it only continues to "exist" because it is really the boys' fear.
For me, one possible quote for this idea comes at the very end of Chapter 8 -- I don't know if we have the same edition of the book so you should find the page number yourself.
Simon is in his secret place with the Lord of the Flies. It starts talking to him (at least in his mind) and it says to him
“Fancy thinking the Beast was something you could hunt and kill!” said
the head. For a moment or two the forest and all the other dimly appreciated
places echoed with the parody of laughter. “You knew, didn’t you?
I’m part of you?