Okay, I think I finally have time to respond to this question! LOL!
To Kill A Mockingbird by Harper Lee, because it had a profound effect on me when I first read it at ten years old. It still does. It's so cleverly written - from the POV of a six year old girl, yet the adults in the book and the reader know exactly what is going on. It's also a thought provoking slice of not-so-great American history. I think it's important that we not forget these things, lest we repeat the mistakes.
The Great Gatsby by F. Scott Fitzgerald, simply because I adore the story. Another slice of American history, plus intrigue, deception, two truly horrible main characters (Daisy and Tom), and a narrator whom you just want to reach out to hug and slap upside the head at the same time.
Little Women by Louisa May Alcott. This is another one I first read at a young age and it had a profound effect on me. I fell in love with the characters from the first few pages. If a writer can do that for me, she/he has me hooked for the entire book.
Everything by Jane Austen, because she's Jane and no one else writes like her. The satire, the detail, the fabulous characters. Of her six published novels,
Mansfield Park is my favorite.
1984 by George Orwell. Scary because it's still so relevant. Also depressing as hell, but I can't stop reading.
Stephen King. Yeah. Everything he writes. There are some stinkers, but I read them all anyway at least once. My favorites are
The Shining, The Shawshank Redemption, The Green Mile, The Eyes of the Dragon, Needful Things, Bag of Bones, Hearts In Atlantis, and
The Stand.
The Hunger Games trilogy by Suzanne Brockmann, because these books are so well-written. I don't read much YA dystopia, but this is one series I'm glad I did. I adore them.
A Rose In Winter by Kathleen Woodiwiss. Classic bodice-ripper romance at its best.
It's a heart-melting story about redemption and trust. I have a VERY worn paperback copy and really should replace it one day.
Sweet, Savage Love by Rosemary Rogers. She was one of the first queens of the bodice-ripper, and this book is the first romance I ever read. I was a tender 19 years old and I was hooked for life on the genre.
Out of Africa by Isak Dinesen. Forget the movie, although I can't completely dislike anything with Meryl Streep in it. The book is phenomenal. What a voice this author has!
Echoes by Maeve Binchy. For that matter, EVERYTHING she wrote.
Echoes was the first book of hers I discovered and I was so hooked I found and bought them all.
The Other Side of Midnight by Sidney Sheldon, again simply because I love the story and the characters.
Ceremony of the Innocent by Taylor Caldwell. Another story that pulled me right in. I couldn't stop reading.
There are others, but those are the ones that spring to mind immediately when someone asks this question.