This will be an unusual (not to mention) obscure choice for most. Mysterious Island by Jules Verne is the only book that I have read more than 3 times in my life, and I have read it over ten. To me, it is the most essential story of the human struggle to survive ethically in a hostile environment that has ever been written.
On the literal level, it is a Robinson Crusoe type of survival story with the usual elements: stranding on a desert island; physical hardships and danger; unseen benefactors; life-or-death decisions between expediency and morality, just to name some. But the master Verne is at his best here by restraining the fantastical elements of his typical work and delving into the minutia of 5 men (later to become 6) who put aside their regional cultural and philosophic differences to create a miraculous Utopian environment in just a few years.
Verne infused Island with his beliefs about pragmatism, racial equality and the need for civilization to always oppose and resist barbarism. The lead character of Captain Cyrus Harding is one for the ages, a man who can produce gunpowder from carcasses and build windmills with no modern tools, yet is equally at home in discussing fine literature and politics. I recall as a child wishing for a Harding-like mentor in my life, and even now my adult inner child looks askance at the American political scene and hopes for one such as his mythic figure to burst upon it and right all of the terrible wrongs done by far weaker, more nefarious men in power.
I first read this at the age of 10, and I just finished my most recent return to it last year at the age of 48. Now, I can see where the writers of the show Lost received at least the core of their ideas about the reactions of humans stranded accidentally on a deserted island. Verne has long been known as one of the fathers of the science-fiction genre, but it is the human non-technical flavor to Mysterious Island that left such an indelible mark on my psyche. Heartily recommended to all - young and old.