Objectivists consider Ayn Rand's ethical theory based on self-interest her greatest contribution to modern philosophy. I am far from subscribing to this view. I do not even consider she has made any significant contribution to the theory of capitalism and liberalism. In this area she seems to me as much Kantian as Mises and Hayek and others, treating life and society as if God did not exist.
The greatest contribution of all by Rand to modern philosophy was to uncover the role of Kant in the history of ideas. According to Rand, the influence of Kant was catastrophic. Chiefly among Kant's errors is his subjectivistic epistemology. As her disciple Leonard Peikoff put it:
"Kant's theory of concepts, accordingly, though not nominalistic, is akin to nominalism in the respect relevant here: both theories represent subjectivist reactions to the deficiencies of intrisicism. In form and scale, of course, Kant's subjectivism is unprecedented. This is what turns Kant a turning point, not merely another skeptic".
(in Objectivism - The Philosophy of Ayn Rand, N. York: Penguin Books, 1991, p. 467)