Once again, this is according to Google Scholar, rounded to the nearest 100. Email me with corrections and omissions. I list only those with at least 3,000 citations, and exclude works of applied ethics. I could not find any results for Harman’s The Nature of Morality on Google Scholar, but I imagine it would make this list.

1. Alasdair MacIntyre, After Virtue, 37,800
2. Derek Parfit, Reasons and Persons, 17,900
3. John Stuart Mill, Utilitarianism, 14,300
4. G.E. Moore, Principia Ethica, 12,100
5. T.M. Scanlon, What We Owe to Each Other, 10,600
6. Bernard Williams, Ethics and the Limits of Philosophy, 10,200
7. Martha Nussbaum, The Fragility of Goodness, 8,800
8. J.L. Mackie, Ethics: Inventing Right and Wrong, 8,500
9. Henry Sidgwick, The Methods of Ethics, 7,800
10. W.D. Ross, The Right and the Good, 7,500
11. Virginia Held, The Ethics of Care, 6,500
12. Alasdair MacIntyre, Whose Justice? Which Rationality?, 6,500
13. Christine Korsgaard, The Sources of Normativity, 6,400
14. R.M. Hare, The Language of Morals, 6,000
15. Iris Murdoch, The Sovereignty of Good, 5,200
16. Michael Smith, The Moral Problem, 4,400
17. Allan Gibbard, Wise Choices, Apt Feelings, 4,200
18. R.M. Hare, Moral Thinking: Its Levels, Method and Point, 4,100
19. Joseph Raz, Practical Reason and Norms, 4,100
20. Thomas Nagel, The Possibility of Altruism, 4,000
21. Ronald Dworkin, Justice for Hedgehogs, 3,800
22. Derek Parfit, On What Matters, 3,300
23. Stephen Darwall, The Second-Person Standpoint, 3,200
24. R.M. Hare, Freedom and Reason, 3,200
25. Philippa Foot, Virtues and Vices, 3,100
26. Charles Stevenson, Ethics and Language, 3,100
27. David Brink, Moral Realism and the Foundations of Ethics, 3,000