I think I would have to choose Cinderella's Royal Table at the MK, even though I haven't eaten there, I have heard good things. None of the rest of the restaurants in the MK at which we have eaten have had any appeal to me. We do like Pecos Bills for a quick bite... their fixin's bar is the best!
At Epcot, we like Le Cellier for dinner. Yes, a bit pricey, but well worth it.
At MGM we like so many of the restaurants, it is difficult to only choose one. For fun, good home cooked food and humor, we would go to 50's Prime Time Cafe ( we do this every trip), for a more sedate sitdown meal I would choose Brown Derby, at lunch... get an early PS. For something middle of the road, Mama Melrose is our choice.
At Animal Kingdom other than Rainforest Cafe, there really aren't any sitdown restaurants, unless you are looking for a character meal at Restaurantosaurus ( which I have heard is good). Our favorite place to eat is Tusker House, which has great chicken dishes, indoor, or outdoor seating and it very reasonable.
As far as eating at the resorts, one of the best values we have found is Cape May Cafe at the Beach Club for dinner.
Bon Apetit!