I have to go with Wintergirls by Laurie Halse Anderson.
I’m already a big fan of Andersons – Speak is one of the best YA books I’ve ever read – and Wintergirls has that same raw, powerful, gut-wrenching voice. The way Anderson captures the aching loneliness of a teenager who has disappeared inside herself is amazing. It’s about a girl battling anorexia and some parts are almost painful to read in their realism. (and you know I love my realistic fiction) It’s one of those “this should be required reading” books for both teens and parents.
Honorable mention: Itch by Michelle D. Kwasney. Kwasney also does a really great job of capturing that lonely voice with her MC, Itch, who is forced to move to Ohio from Florida after the death of her beloved grandfather. It’s almost the things left unsaid that are the most touching in this book.