A Theory of Justice

Rawls re-defined political philosophy. You don't need to agree with his answers but the way he framed the discussion made political philosophy respectable again and set the stage for the future discussions (e.g., Nozick). This is a hard book to finish — not so much because it's poorly written or dense (like some continental philosophers...) but because Rawls painstakingly goes through each possibility and systematically discusses both sides of every issue.