The Constitution of Liberty can in many ways be regarded as Hayek's most important work. It centres around a highly nuanced defence of the free market based upon the concept of spontaneous order. But in articulating this defence Hayek skilfully interweaves philosophical and historical insights, at all times displaying tremendous erudition and learning.
The result is not a comprehensive treatise like von Mises's Human Action, or the Wealth of Nations. Hayek always regarded himself as an intellectual 'muddler' (albeit a brilliant one). And his work reflects this. I prefer him when he is focused upon a particular issue at hand, like in The Road to Serfdom, or in the essays that comprise Individualism and the Economic Order.
This is not to detract from the value of this work. Hayek's defence of negative verses freedom, his description of the rule of law, the chapter on Responsibility and Freedom, and the post-script 'Why I am not a Conservative,' deserve to be treasured. Hayek's intellectual integrity shines through here. He was never a polemist or an extremist, and this has compromised his stature amongst libertarians. But Hayek's particular position on any single issue should not be of concern here. A reiteration of classical liberalism will always be of value, but this work stands out for the subtlety of its insights, and the range and depth of its arguments. Hayek's ideas should be recognised as providing, along with those of Mises and Milton Friedman, the best twentieth century defence of a free and spontaneously ordered society; a defence which should be distinguished from the limited and compromised one provided by many neo-classical economists, by social democrats or conservatives, and the dogma provided by Aynn Rand and her disciples.