I list all those with at least 1,500 citations (rounded to nearest 100 as usual), since numbers drop off more quickly here (philosophy of law is a lower citation field than political philosophy).
- H.L.A Hart, The Concept of Law, 28,000
- Ronald Dworkin, Taking Rights Seriously, 26,300
- Ronald Dworkin, Law’s Empire, 12,800
- John Finnis, Natural Law and Natural Rights, 10,800
- Lon Fuller, The Morality of Law, 9,900
- H.L.A. Hart, Punishment and Responsibility, 6,200
- Joseph Raz, The Authority of Law, 5,300
- Jeremy Waldron, Law and Disagreement, 4,700
- Joseph Raz, Practical Reason and Norms, 4,100
- H.L.A. Hart & Tony Honore, Causation in the Law, 3,800
- H.L.A. Hart, Law, Liberty and Morality, 3,400
- Frederick Schauer, Playing by the Rules, 2,700
- Allen Buchanan: Justice, Legitimacy, and Self-Determination: Moral Foundations of International Law, 2,500
- Joel Feinberg, Harm to Others, 2,300
- Ernest Weinrib, The Idea of Private Law, 2,300
- Jeremy Waldron, The Harm in Hate Speech, 2,000
- Joel Feinberg, Doing and Deserving, 1,800
- Joel Feinberg, Harm to Self, 1,700
- Michael S. Moore, Placing Blame: A Theory of Criminal Law, 1,700
- Joseph Raz, The Concept of a Legal System, 1,700
- Jules Coleman, Risks and Wrongs, 1,500
- H.L.A. Hart, Essays in Jurisprudence and Philosophy, 1,500
With the proviso that I’m no longer in the field and that it is a book, not a course or…