Chicago, IL -- After years of seeming that he didn't like Jay Leno, David Letterman now says he's "the funniest guy" he's ever known. He tells Oprah Winfrey that Leno's nightclub act is "the smartest." He calls Leno "a wonderful observationist and very appealing as a comic." At the same time, Letterman says Leno might also be "the most insecure person" he's ever known. The interview airs Sunday on OWN.