To Kill a Mockingbird by Harper Lee
Grapes of Wrath by John Steinbeck
Zen and the Art of Motorcycle Maintenance by Pirsig (sp?)
Lord of the Flies by William Golding
The Little Prince by Antoine de Exupery (sp?)
Picture of Dorian Gray by Oscar Wilde
Book of Everything by Guus Kuijer
Life of Pi by Yann Martel
Stranger by Albert Camus
Brave New World by Aldous Huxley
Fahrenheit 451 by Ray Bradbury
Atlas Shrugged / Fountainhead / We the Living by Ayn Rand
Screwtape Letters by CS Lewis
Okay, these aren't necessarily YA but I enjoyed many of these in high school and the others I discovered as an adult. I suspect you've read these too.
I was thinking that the timeless books are all philosophical in nature and look at this goldmine of resources (PBs):
http://www.teachingchildrenphilosophy.org/BookModule/BookModule Happy reading. Is this for a class you are developing?