When you sit down with somebody for an hour, you get a real sense of who they are,'' says Oprah Winfrey in her running commentary on The Oprah Winfrey Show: 20th Anniversary Collection. And yet it isn't the stars (John Travolta and the Judds among them) that make the set worth watching. Most interesting is learning which guests, like Elie Wiesel, meant the most to Oprah, and why. And of course, there's her ever-evolving appearance ''For me, good taste was a long time in coming,'' she says cheerfully especially her frank discussions about her weight. Recalling her most popular segment, 1988's ''The Fat in the Wagon'' where she fit into a pair of size 10 jeans after four and a half months on a liquid diet she admits, with a twinge, that she couldn't zip them up just two days later. But it's exactly this humanness that makes her viewers able to relate to her, and her, in turn, able to inspire them.