I've recommended it before, but I continue to be amazed by the consistently high quality and erudition of the essays in the Claremont Review of Books. If I were to cut down my subscriptions to one journal, this would be it. 'nuff said.

UPDATE: I should point out that the Review is not a libertarian publication, but promotes its own idiosyncratic brand of natural-rights-based conservatism, which is apparent in some, but not all, of the essays it publishes. But I don't have to agree with every essay to appreciate the quality of the writing and editing.