The correct answer to this open question is the following.
The two ideas debated by the philosophers of the Enlightenment that I believe to be the most important natural rights of an individual in a free society are the right to property and the idea of limited government.
In the first case, one of the most important natural rights for men is the possibility to work hard to have their own patrimony: a house, land, personal possessions. That is why people work hard, to later enjoy the "fruit" of their work.
In the second case, limited government, because people have the right to choose their rulers. That is why they participate in elections. And they also have the right to remove bad politicians that do not perform well ruling the government. Also, people do not like tyrants.
John Locke was one of the great thinkers and philosophers of the European Enlightenment with other thinkers like Baron de Montesquiou, Voltaire, Jean-Jaques Rosseau, and Thomas Hobbes.