I don’t imagine there is one answer, but if I am required to choose one it would be Rothbard.
I can accept that Rothbard’s views on anarchy are not acceptable to all libertarians, as many libertarians hold on to the idea of a minarchist state. However, it seems to me that some bare minimum requirement is necessary to be seen as a “real libertarian.”
First and foremost, if a government does anything more than protect the personal property of those within its jurisdiction, it is no longer a government but a well-funded and well-organized thief. If not for the absolute protection of private property, why would men voluntarily form together? They certainly do not form together to have their property extorted!
For those who advocate exceptions to this, no matter how seemingly trivial, the label libertarian cannot be applicable. No helping the poor, no foreign aid, no unemployment benefits, no price controls, no trade barriers, no funding basic science and research, no public education, no central banking, no overseas bases, no “liberating” other people, no funding of highways and roads, no metropolitan water districts, no “natural monopolies” (only enforceable by the state). None of it.
Second, the same rules must apply to all. A badge and a gun do not make one right in situations where others without such adornments are wrong. The badge and gun cannot absolve murder or theft. Any advocacy counter to this would render the libertarian label as meaningless.