Mine would be Good Omens by Neil Gaiman and Terry Pratchett. Its a screamingly funny satire about the Apocalypse and Armageddon.
I can't name my favorite quote because there are just too many of them, but one of them is "[Humans] were born into a world that was against them in a thousand little ways, and then devoted most of their energies to making it worse... Just when you'd think they were more malignant than ever Hell could be, they could occasionally show more grace than Heaven ever dreamed of. Often the same individual was involved. It was this freewill thing, of course. It was a bugger."
My favorite quote from Good Omens - which is an absolutely wonderful book that all should read - is about the protagonist's father. For those who haven't read it, the basic plot is that Armageddon is coming because the Anti-Christ has finally been born. He will be raised by a carefully chosen family in order to come into his full power and evilness and thus eventually cause the end of the world.
Only, there's a mix-up at the hospital and by accident he's given to a normal family, the Youngs. The Young raise him as best they can, so the boy we end up reading about is what happens when someone with the potential to be evil incarnate is raised by decent, caring people. It's all about the freewill thing, as Two Ravens said.
Anyway, my quote, about Adam's father, probably the greatest influence on the boy born to bring hell and damnation into the world:
"Archimedes said that with a long enough lever and a solid enough place to stand, he could move the world.
He could have stood on Mr. Young."
That's who I want raising the spawn of Satan. That man there.
