This one is going to be interesting. I would say that Rousseau's biggest achievement is to be one of the first voices of intellectual complexity. Contrary to other thinkers in the Enlightenment period, Rousseau was simply complex and intricate in his thinking. Voltaire was almost monolithic in his assertion for individual rights, while Montesquieu's principles of divided government helped to define him in many spheres. Yet, Rousseau was complex, writing on a great many topics and evoking the thorny and challenging elements within each domain. For example, Rousseau writes about the need for the general will in political systems, yet he also understands the need for individual liberties in this system to prevent a dictatorship. At the same time, Rousseau understands that individual freedom at the cost of progress and advancement in a political setting could result in disaster. One sees the same development of thought in his writing on education. While embracing the Enlightenment theory of acquiring knowledge, he was one of the first thinkers to suggest that the best way to ensure the proper intellectual growth of a child is to ensure that individualized approaches to knowledge acquisition should feed the development of the "whole child," a premise that flew directly in the face of Enlightenment theories. I would say that Rousseau's biggest achievement is the idea that thinkers can be complex and raise different aspects of intellectual development in embracing ideas and developing the historical narrative of ideas.