I have heard that "Boatwrights" at the Port Orleans Riverside Resort is known for their prime rib. I have not tried it myself, so I can't say for sure. I know it's on the menu there. We have a reservation for Boatwrights on our next visit.
The best prime rib I've had at a Disney restaurant was from the Yachtsman Steakhouse. That was a very good quality cut of beef. I've had the prime rib from Boatwrights, and it is okay, in fact, its not too bad. But the cut of beef was better at the Yachtsman.