I'd say it was Christina Hoff Sommers, this past Wednesday, promoting her book about how the self-esteem movement in schools has gone too far. The tales she had to tell -- about "Circle of Friends" replacing tag and the demand that the red correction pen give way to lavender -- really connected with the audience, who seemed quite eager to receive this info. They were prepped, I think, by some combination of their own school experiences and "South Park." And Stewart did not make it easy for her. His approach was to challenge whether her little anecdotes represented anything other than some highly affluent enclaves. Every time he made this move, she shot out some little thing -- "The Girl Scouts have an anti-stress badge" -- that made the audience burst into spontaneous laughter. And you know it would not be their normal thing to give a warm welcome to a right-ish guest.
Regrettably, the Sommers interview is not included on the best interviews page at the "Daily Show" site. (You can, however, see the Dennis Miller interview. He, too, did well. I felt bad for him when he expressed his envy of the "Daily Show's" audience, which was, apparently, visibly way hipper than the crowd he's stuck with out in California.)