Les Mierables is influenced by opera, so anything that can show off your legit voice would be great. And most legit songs (early era) are written for the ingenue, like Cosette. Sing a song that will reflect her innocence. But I'd avoid Phantom--it's too operatic and too overdone. I strongly suggest singing a ballad, because Les Mis is Ballad Heaven. Check out the repertoire of Shirley Jones and Julie Andrews. I'm a Soprano myself, (6 octaves, baby!) these are from my own repertoire:
A Dream Is A Wish Your Heart Makes - Cinderella (No one ever does the simple classics anymore. But this song is not as easy as it sounds, difficult intervals.)
Before I Gaze At You Again - Camelot
I Have Dreamed - The King and I
If I Loved You - Carousel
Out Of My Dreams - Oklahoma!
This Can't Be Love - Boys from Syracuse
Till There Was You - The Music Man
Break a leg!
Musical Theatre Enthusiast, Soprano
my own repertoire