A few essentials anyone should read if they can.
Beyond Good and Evil, Nietzsche
Nausea, Sartre
Ethics, Moore
The three critiques (Practical Reason, Pure Reason, and Judgment), Kant - I recommend starting with Judgment and ending with pure reason
The Prince, Machiavelli
The Republic, Plato
Euthphyro, Plato
Leviathan, Hobbes