Any recommended reading list is going to be subjective and possibly controversial. I've included here books which I personally believe are worthwhile or even, in some cases, necessary reading for a young adult. Admittedly, the worth of some is primarily in the conversation they ought to spark between you and your teenager, and I've noted where I think this is true.
I've also excluded some books because my daughter and I personally didn't find them worthwhile, or because they go against my own values (such as books which are unpleasantly manipulative or contrived), or simply because I haven't read them yet. And some of your favourite young adult books may be missing because my daughter read them as a tween, and so they've fallen off my radar, such as anything written by Madeleine L'Engle.
I haven't read much of this genre, so my recommendations will be brief.
On The Jellicoe Road, by Melina Marchetta.
Saving Francesca, by Melina Marchetta.
Looking for Alibrandi, by Melina Marchetta.
Wanderlove by Kirsten Hubbard.
Because of Winn-Dixie, by Kate DiCamillo
Crossing the Tracks, by Barbara Stuber.
The Secret Life of Bees, by Sue Monk Kidd.
The Curious Incident of the Dog in the Night-time, by Mark Haddon.
Flyaway, by Lucy Christopher.
Stolen, by Lucy Christopher
Fantasy & Sci Fi Fiction
The Hunger Games series by Suzanne Collins. Very violent but it should be followed by discussion.
The Princess Bride, by William Goldman.
Ender's Game, by Orson Scott Card. I don't rate it highly but it leads to good discussion.
The Giver, by Lois Lowry
Gathering Blue, by Lois Lowry
Number the Stars, by Lois Lowry
Son, by Lois Lowry
The Tiffany Aching series, by Terry Pratchett
The Discworld series, by Terry Pratchett
Feed, MT Andersen (with reservations)
The Dark is Rising, by Susan Cooper
Unwind, by Neal Shusterman. Violent but worth discussing.
The Song of Wirrun, by Patricia Wrightson.
The Dragonsong series, by Anne McCaffrey.
Dragonflight, by Anne McCaffrey (with reservations, contains old-fashioned sexism.)
The Shapechanger's Wife, by Sharon Shinn
The Chrysalids by John Wyndham.
The Tripods trilogy, by Samuel Youd
Beauty, by Robin McKinley.
The Blue Sword, by Robin McKinley
The Hero and the Crown, by Robin McKinley
The Last Unicorn, by Peter Beagle
Juniper, Wise Child & Colman, by Monica Furlong.
The Wizard of Earthsea series, by Ursula Le Guin.
The Lord of the Rings, by JRR Tolkien.
Dune, by Frank Herbert.
Tuck Everlasting, by Natalie Babbitt.
The Thief series by Megan Whalen Turner
Anything written by Connie Willis, including her short stories.
Anything written by Patricia McKillip.
Many books in this category which I hear recommended to teens, such as My Side of the Mountain, I consider to more rightly belong in a tweens' list. That may be a consequence however of running a literature-based homeschool!
The Scarlet Pimpernel by Baroness Orczy
Little Women, by Louisa May Alcott
Great Expectations, by Charles Dickens.
A Tale of Two Cities, by Charles Dickens
David Copperfield, by Charles Dickens.
Huckleberry Finn, by Mark Twain
The Trial, by Franz Kafka.
Metamorphisis, by Franz Kafka.
Brave New World, by Aldous Huxley.
To Kill A Mockingbird, by Harper Lee
The Hound of the Baskervilles by Arthur Conan Doyle
The One & Future King, by TH White.
Animal Farm by George Orwell
Lord of the Flies, by William Golding.
The Anne of Green Gables series, by LM Montgomery
The Emily Climbs series by LM Montgomery
The Blue Castle, by LM Montgomery
Betsy-Tacy series, by Maud Hart Lovelace.
The Three Musketeers, by Alexandre Dumas.
The Count of Monte Cristo by Alexandre Dumas.
I Capture the Castle, by Dodie Smith.
Catcher in the Rye, by Holden Caulfield.
Catch-22, by Joseph Heller
The Phantom of the Opera, by Gaston Leroux.
The Crucible, by Arthur Miller. (Actually a play)
The Importance of Being Earnest, by Oscar Wilde (also a play)
A Tree Grows in Brooklyn, by Betty Smith.
The Screwtape Letters, by CS Lewis.
The Island of Blue Dolphins, by Scott O'Dell
Everything written by Jane Austen.
Jane Eyre, by Charlotte Bronte.
Agnes Grey, by Anne Bronte.
Les Miserables, by Victor Hugo
Moby Dick, by Hermann Melville.
The Scarlett Letter, by Nathaniel Hawthorne.
Jayber Crow, by Wendell Berry.
Fahrenheit 451, by Ray Bradbury.
The Call of the Wild, by Jack London.
Utopia, by Thomas More.
Hamlet, by Shakespeare. (And all of Shakespeare if you want!)
I understand the belief that some teenagers need dark books which deal unstintingly with the reality of their unhappy lives. However, having read many of these books, I find many fail to lift up the spirit and encourage a reader to trust that the world is essentially a good place. I believe that, instead of reading "gritty, dark and realistic" fictional stories which may or may not deal carefully enough with controversial subjects, it's perhaps more beneficial for a young person to read real-life accounts of young men and women who suffered and found hope, beauty, and courage in the shadows of their life. Here are some examples ...
The Diary of Anne Frank, the definitive edition, by Anne Frank.
The Story of My Life, by Farah Ahmedi.
Within Reach: My Everest Story, by Mark Pfetzer.
Do Hard Things, by Alex Harris.
Thura's Diary, by Thura Al-Windawi.
Beautiful Days of My Youth, by Ana Novac.
Soul Surfer, by Bethany Hamilton.
Rachel's Tears, by Darrell Scott.
Room to Grow, by Tracy Gold.
Devil In The Details, by Jennifer Traig.
Needles, by Andi Dominick.
Breaking Through, by Francisco Jimenez.
They Stood Alone, by Sandra McLeod Humphrey.
Growing Up In Slavery, by Yuval Taylor.
Code Name Verity, by Elizabeth Wein. (A novel, but difficult themes.)
Read all the great poets lavishly!
So many biographies are as exciting as novels, as well as being edifying and educational. Here is a list of good biographies to inspire you. A favourite of mine is Nicholas & Alexandra by Robert K Massie.
Read the great speeches of people throughout history, from Queen Elizabeth to Martin Luther King and more.
Read Plutarch, Pliny, Caesar, Homer, and so on, to develop an appreciation of the foundation of our civilisation. Read literature, letters, and essays from the Renaissance and Victorian eras. For example, the letters of Henry VIII to Anne Boleyn are insightful, especially if you subscribe to the theory that she did not want to marry him. The letters of Keats are a revelation. The diaries of many gifted authors are also interesting reading.