You can find a ton of info on these boards if you look in the Dining section, but I'll give you my humble opininon:
MK - Food - Crystal Palace
Atmosphere - Cinderella's Royal Table (nothing like eating in the castle)
Epcot - Food - Depends on your ethnic tastes, I like Biergarten (good buffet)
Atmosphere - Coral Reef
MGM - Food - 50's Prime Time Cafe' or Hollywood Brown Derby
Atmosphere - 50's if you like that kind of thing or Sci Fi
AK - Food - can't go wrong with Tusker House
Atmosphere - Rainforest or Donald's Breakfastosaurus (good buffet too)
DTD - Food - for the price, Earl of Sandwich is really good and filling
Atmosphere - Planet Hollywood or Rainforest