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).

1. H.L.A Hart, The Concept of Law, 28,000
2. Ronald Dworkin, Taking Rights Seriously, 26,300
3. Ronald Dworkin, Law’s Empire, 12,800
4. John Finnis, Natural Law and Natural Rights, 10,800
5. Lon Fuller, The Morality of Law, 9,900
6. H.L.A. Hart, Punishment and Responsibility, 6,200
7. Joseph Raz, The Authority of Law, 5,300
8. Jeremy Waldron, Law and Disagreement, 4,700
9. Joseph Raz, Practical Reason and Norms, 4,100
10. H.L.A. Hart & Tony Honore, Causation in the Law, 3,800
11. H.L.A. Hart, Law, Liberty and Morality, 3,400
12. Frederick Schauer, Playing by the Rules, 2,700
13. Allen Buchanan: Justice, Legitimacy, and Self-Determination: Moral Foundations of International Law, 2,500
14. Joel Feinberg, Harm to Others, 2,300
15. Ernest Weinrib, The Idea of Private Law, 2,300
16. Jeremy Waldron, The Harm in Hate Speech, 2,000
17. Joel Feinberg, Doing and Deserving, 1,800
18. Joel Feinberg, Harm to Self, 1,700
19. Michael S. Moore, Placing Blame: A Theory of Criminal Law, 1,700
20. Joseph Raz, The Concept of a Legal System, 1,700
21. Jules Coleman, Risks and Wrongs, 1,500
22. H.L.A. Hart, Essays in Jurisprudence and Philosophy, 1,500