For me, Cosette has always been the character in Les Miz who is the palest adaptation of her character in the novel. In Hugo's novel, Cosette is an innocent, charming young girl who is on the verge of womanhood--utterly lovely in every way. In the musical, Cosette comes across (at least to me) as a bird brained half-character who serves little purpose at all.
That said, the material presented is so weak, there's not much hope for an actress to really do much with the role than stand there and look/sound pretty. Rebecca Caine on the OLC and Judy Kuhn on the OBC probably come the closest of making me care for Cosette. The best I can explain is that they actually attempt to show motivation for the character--Kuhn provides emotional dimension and heart to the character. Caine evokes the loveliness of the girl-woman from the novel.
As for the Cosettes I've seen, none of them stand out to me. None was any better or any worse than the other. As long as you can stay on pitch and not sound too shrill, you've done an acceptable job.
"You're the worst thing to happen to musical theatre since Andrew Lloyd Webber!" --Family Guy
"Shut up! It's been 29 years!!!" --the incomparable Patti LuPone in her MUCH DESERVED Tony acceptance speech for Gypsy.
Kitzy's Avatar du Jour: Kitzy as Little Red Ridinghood in her college's production of "Into the Woods"