Looking over Greg's list, there's no argument that Thomas Sowell deserves his No. 1 ranking. Ever since this blog started, Sowell's The Vision of the Anointed has been on the sidebar as the single best volume for those who want to understand modern liberalism. It's possible to view The Vision of the Anointed as an expansion on the theme developed by Hayek in The Mirage of Social Justice. What I've always admired about Sowell is the admirable clarity of his writing. He has an excellent knack for boiling things down to an understandable essence.
John Stossel at No. 2, George Mason University's Walter Williams at No. 3 -- both excellent choices. It is possible to argue that Williams's influence at GMU, where he teaches the school's Ph.D. candidates in economics, makes him the single most influential free-market educator in America. GMU is also home to the Mercatus Center, which helps nurture free-market ideas.
If I could quibble with Greg's list, I'd suggest he take another look and consider adding Michelle Malkin. Since 9/11, Michelle has been pigeonholed as a "neocon" hawk, but she has a strong free-market background -- she was a Brookes Fellow at CEI -- and has been perhaps the blogosphere's most prominent opponent of the Keynesian stimulus/bailout approach to the economic crisis.