It's one of the best kept secrets in all of entertainment: who will be Oprah Winfrey's final guest, ever? With only 20 days left until the the May 25 finale, friends of the talk queen hint that the leading candidate at the moment is the biggest star in the world ... Oprah herself!

"How can one celebrity possibly sum up 25 years of such a groundbreaking show?" an insider asks. "Who could possibility come on the show and represent everything Oprah has done" from celebrity interviews, to human-interest stories to politics to women's issues to breaking news.