Tyler Cowen asks an excellent question: Excluding economists (i.e. no Milton Friedman), which 20th century classic of American conservative political thought holds up best?
Cowen suggests that few look all that good in hindsight - "mostly because they underestimated the robustness of the modern world and regarded depravity as more of a problem than it has turned out to be." A slightly different way of looking at the matter is that many of the classics of American conservative political thought were written when Communism seemed to be in the ascendancy worldwide, and when ever-increasing-statism, at the very least, was assumed by nearly every intellectual to be the wave of the future. Small wonder, then, that they have a tendency to view all of Western politics through the lens of creeping Marxism; small wonder, too, that they can seem alarmist and apocalyptic in hindsight, with their anxieties about "the totalitarian implications of the federal school lunch program," and so forth. (If Orwell's 1984 were a treatise of political philosophy, rather than a novel, it would have a similarly fusty and irrelevent air today. All that stuff about boots stomping on the human face forever - so over-the-top!). The fact that a lot of mid-century conservative writing doesn't seem to hold up well today is, at least in part, a testament to how complete the anti-Communist victory has been - and also how unexpected.
As for books and authors that do hold up - well, Cowen says that we can include grumpy European emigres, so I'd have to vote for the collected works of Leo Strauss (you can't pick just one!), who should be required reading whatever you think of his disciples' influence on contemporary politics. Speaking of his disciples, I'd also vote for Allan Bloom's Closing of the American Mind. (Contrary to what you may have heard, the second half of the book is better than the first: The critique of student life is curmudgeonly; the critique of the academy as a whole is brilliant.) A variety of neoconservative works, from Edward Banfield's The Unheavenly City to Charles Murray's Losing Ground to almost anything by James Q. Wilson, all hold up quite well in hindsight, and so do the classic Catholic-neocon books, Richard John Neuhaus's The Naked Public Square and Michael Novak's The Spirit of Democratic Capitalism. (Novak has been rather too carried away over the years by his enthusiasm for the affinity between Christianity and capitalism, but his early writings on the subject are very good.) And while I know Francis Fukuyama no longer calls himself a neocon, his The End of History and the Last Man remains a brilliant diagnosis of the politics of late modernity, however its prognosis ends up being remembered.
Depending on how flexible we're willing to be with the definition of "conservative" on the one hand and "political thought" on the other, I'd also put in for Christopher Lasch (The Revolt of the Elites especially, but also The True and Only Heaven), Philip Rieff (particularly The Triumph of the Therapeutic), Tom Wolfe (both in his nonfiction and in Bonfire of the Vanities), and the young Joan Didion of Slouching Toward Bethlehem and The White Album, before she went east, and downhill.
So that's a start. Meanwhile, Ezra poses the same question for liberal political thinkers.