Beautiful and enchanting. She has a delightful ethereal quality to her. She could be the Lady of the Lake's sister or an entirely new enchantress. The sword she's holding is amazing. A blade of evil's bane, to instill hope in the hands of its wielder as they fight to keep the light of hope alive. Exceptional work. The detailing and the lighting is perfection. Her gown is gorgeous and the forest around her is astonishing. A true work of art.
I would say it's his lady, Queen Guinevere. And if not her then Camelot, in some mythology the ideals of his fabled kingdom were most dear to Arthur. Though getting back to Gwen, what is a king and his kingdom and it's ideals without his queen supporting him?
Those are very good answers. Indeed, would say those things are worth even more than Excalibur, being priceless. But what is the one thing of exact equal value?
A hint: It is a trick question - and the Lady of the Lake implies it.