Asking for "Best Philosophical Books" is pretty much like asking for an opinion, everyone will have a very different view. That being said, here's my view!

John Stuart Mill - "Utilitarianism" (Kant be damned.)

Thomas Nagel - "Moral Luck"

David Lewis - "On the Plurality of Worlds"

Plato - "The Republic"

Aristotle - "The Metaphysics"