The end is near. After 25 years, The Oprah Winfrey Show's last episode is set to air on May 25. But there's one big mystery left: Who will the queen of talk's final guest be? Here, five theories:

1. Stedman Graham
Oprah has long kept her romantic partner, Stedman, out of the spotlight. But that could change with the finale, says Kate Ward in Entertainment Weekly. "I love the idea of Graham as a guest." He would offer the audience some new insight and fresh perspective into who Winfrey really is.

2. Gabrielle Giffords
The recovering congresswoman is another likely possibility, says Ward. She's a classic Oprah guest — "an influential, well-known headline-maker with a touching story that ends on an uplifting note."

3. One of Oprah's famous friends
"Winfrey has a stable of celebrity friends who have appeared repeatedly on her talk show," says Caryn Rousseau for the Associated Press. Tom Cruise, Katie Holmes, and Jennifer Aniston are all possibilities, as are world leaders like Nelson Mandela and Bill Clinton (the Obamas were already on last week). Or maybe a "well-respected" old friend like Maya Angelou, Sidney Poitier, or Barbara Walters.

4. One of Oprah's famous protégés
Maybe Oprah will have on some of the celebrities she helped rocket to "their own television stardom," says Rousseau. We're looking at you, Dr. Phil and Nate Berkus.

5. Oprah herself
My sources says the "biggest star in the world" is the leading contender for the final guest slot, says Rob Shuter at PopEater. Yes, that means Oprah herself. Insiders say it makes sense. She's the only one capable of summing up all the show has been and done in 25 years.