To whom is the lady being compared to in the first stanza in "She Walks in Beauty"?

Byron uses natural elements as a means to both articulate
and compare the subject's beauty.  Consistent with Romanticism, Byron uses nature as the
representation of perfection.  This makes its comparison to the woman's beauty an aspect
of perfection, something that cannot be replicated.  The "night" and "cloudless climes"
and "starry skies" of lines one and two are merged with the subject, herself, in line
four, when the speaker argues that the subject of the poem is the representation of such
beauty: "Meet in her aspect and in her eyes."

Another form
of comparison that is present in the first stanza can also be seen in line three. 
Romantic thinkers were zealous advocates of being able to merge social and political
elements of the good with a state of beauty.  Beauty was seen as the physical
representation of justice and honor, qualities that are intangible and impossible to
physically define.  Romantic thinkers argued that the aesthetic experience of beauty
also represents these realities.  Recall Keats' assertion of "beaty is truth" in his
work, and one sees how this is a Romantic idea.  Byron also does not miss a chance for
this in line three where the speaker argues that "And all that’s best of dark and
bright" is present in the subject of the poem.  In this comparison, Byron is making the
argument that the subject is a physical representation of the intangible, apotheosizing
her beauty as the physical manifestation of those noble virtues that cannot be
quantified.