Johnny Got His Gun by Dalton Trumbo is easily in the top 10 somewhere. This novel changed my world view. I read it as a high school student and have come back to it again and again.
The Poisonwood Bible by Barbara Kingsolver is also easily in my top 10 somewhere. This novel is beautifully executed and the story is so well told.
For Children's Lit, I can easily put Bridge to Terabithia by Katherine Paterson in the category of favorite books of all times. Even as an adult, I cry when I read this book.
In foreign lit, I have the book that defines Spanish literature, The Shadow of the Wind by Carlos Ruiz Zafon, which is everything I want in a novel. It's modern and beautiful. A perfect read that I have picked up over and over again.
In nonfiction/memoir, I have The Night Trilogy: Night, Dawn & The Accident by Elie Wiesel. This Holocaust memoir speaks for me not just about the experience of the Holocaust, but what it meant for the years afterwards.
I'm still working on the rest. Likely on the list, but not there yet:
Black, White, & Jewish: Autobiography of a Shifting Self by Rebecca Walker in nonfiction
The Giver by Lois Lowry in young adult lit.
The Secret Life of Bees by Sue Monk Kidd in Fiction
Frankenstein by Mary Shelley in Classics
What books would be on your list?