Does it have to be a solo? If not, my first thought was There Is No Other Way from Pacific Overtures. Another one I love is I Read from Passion.
Since you said it doesn't have to be Sondheim, here are some other thoughts:
Fable - Light in the Piazza
Invisible - Women on the Verge
Holding to the Ground - Falsettoland
When I Look at You - Scarlet Pimpernel
Change - A New Brain (very relevant for today's social and political climate)
With One Look - Sunset Boulevard (to me, signing a song about conveying expression with no words is rather poignant)
I Hate the Bus - Caroline, or Change
I Miss the Mountains - Next to Normal
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian