From a (fairly informal) Facebook post of mine:
First significant thinker to get libertarianism totally right: Hans-Hermann Hoppe. Let me splain.
I view the modern libertarian movement as starting around the 50s or so, with people like Leonard Read, Ayn Rand, Milton Friedman, and a bit later, Rothbard, and the like. Yes there were important forbears—Bastiat, classical liberals, and others. (http://archive.mises.org/18385/the-origin-of-libertarianism/; see also Libertarianism After Fifty Years: What Have We Learned?)
But the earlier libertarians always got something major wrong or were missing some major essential points. Most of them were pro-state—not anarchist. Minarchists, classical liberals. That’s a serious problem.
And the ones who were anarchist always seemed to get some major issue wrong. For example, Spooner, who was great on just about everything, was bad on IP (http://c4sif.org/2012/06/tucker-on-spooners-one-flaw/). This flows from a confused concept of the nature of rights and acceptance of the confused idea of the labor theory of property, stemming from Locke’s formulations and overly metaphorical thinking.
Probably the best overall libertarian in pre-modern times was Benjamin Tucker but even he, like lots of the earlier anarchists, was confused on some basic economic issues, the “land” question, etc.—this latter issue even corrupted his heroic opposition to IP: his argument against IP is that it is based on the idea that you own the products of labor (“he who first takes possession of any material production of nature”), but that this would imply you can own land. And we know we can’t have ownership of land, therefore the principle behind IP must be flawed too. http://fair-use.org/liberty/1891/03/21/land-monopoly-and-literary-monopoly
So Tucker was great, esp. for his time, but not complete.
Further, he was too early to benefit from modern Austrian economics, especially the praxeological-Misesian approach. Which I regard as essential to being a basically modern, complete, systematic, coherent, principled libertarian. You need to be anti-state/anarchist, Austrian (Misesian), and also consistent and very propertarian. Without this it is more proto-libertarian or flawed libertarianism.
Rand was bad on IP (a major issue) and bad on the state. So fail.
Milton Friedman—ditto (at least on the state).
Read was great, and good on IP, and Austrian-ish economics, but he was not an anarchist AFAIK.
Hazlitt was getting closer, but as far as I know he was not an anarchist. In any case, he was not a comprehensive political philosopher.
One of the people I’m learning a bit more about is Sam Konkin III. From everything I know about him he was pretty solid on everything—the state, IP, everything. He was in fact one of the pioneers of the modern anti-IP movement. However, he was more of a minor figure and did not have a fully fleshed out political theory that I am aware of. He is known for “agorism” and his fairly brief (but profound and correct and perspicacious) comments on IP, but ….
So the obvious candidate is Rothbard. Anarchist, radical, propertarian, profound, comprehensive and systematic, steeped in Misesian economics. You might award him this crown. But he misstepped on IP. It is not just a misstep that is the issue here; it is why he did it. He failed to apply his own property rules and contract theory consistently here. And the former was, I think, because he did not emphasize the role of scarcity and conflict at the root of property enough. Being an expert on Misesian praxeology, with its emphasis on the role of scarce means in human action, it’s a bit surprising, but hey, you can’t do everything. Every great thinker stands on the shoulders of giants, as Rothbard himself did (including being influenced Rand’s sort of systematic tying together of various strands of thought into a libertarian-ish whole), even as he was a giant in his own right.
Hoppe, thoroughly steeped in both Mises and Rothbard, finally got it right, IMO. He did not write much about IP but in his brief comments he indicated the right path. And he also focused intensely on property rights, and—crucially, the issue of scarcity and its core relationship to property rights. He built on Rothbard and Mises, with insights from people like Hume (scarcity) and others like Habermas (rights theory, argumentation ethics, which Rothbard enthusiastically endorsed and saw the revolutionary promise of). If you combine Mises, Rothbard, and Hoppe, you get the culmination of advanced, consistent libertarian philosophy. (See Hoppe, A Realistic Libertarianism)
Not saying this is perfect or the political philosophy is closed or complete, nor that there is not more work to do. But in my view, the main edifice of modern, radical, principled libertarianism is Rothbardian-propertian-Austro-anarcho-Hoppeanism.