We really liked the Norway and Germany restaurants in Epcot, although they are buffet style (unusually good buffet, however).
The SCI-FI Dine in theater is my favorite at MGM, mainly because of the atmosphere. I've heard some negative comments about the food, but we thought it was very good.
You also might want to think about the Fantasmic Dinner Package. You get a dinner at one of three MGM restaurants as well as easy seating at the Fantasmic show at night. We're going to do this next time we go!
Good luck!