What was Rousseau's biggest achievement?

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.