Like so many of you said, Fedor. There are no other good options. If he wins the Gonzaga fight, they should bring Fedor in the way they brought Hendo in and put both titles on the line. If he loses the Gonzaga fight, bring Fedor in to fight Randy and give the winner of Fedor/Randy a shot at Gonzaga.
Whether Randy wins or loses, he will go out with honor. If he wins, we get to see one of the most important fights in MMA history and Randy gets to retire with the unified UFC/Pride HW belts. If he loses, he can retire, and his image won't be tarnished like Ken Shamrock's and Fedor will be set up to fight Crocop/Nog/etc... for the belt.
It's probably obvious that I think Couture should retire soon. He's my favorite fighter of all time and I don't want to see him go out like Sharock. I'm not too worried about him though. He's too smart to Shamrock himself.