MK: We always eat at Cosmic Rays, sometimes twice in one day
AK: Flametree or Tuskerhouse
Epcot: we are actually partial to the pretzel and hotdog stands that you can find located throughout Futureworld. If you are looking for something more substantial try the Electric Umbrella in Futureworld. They have some interesting stuff like the tossed island chicken salad. In World Showcase try the Liberty Inn.
MGM: for the family who can't decide try these counter service/kiosk options in basically one place: Catalina Eddie's (pizza, salads) Rosie's All American Cafe (burgers, sandwhiches, soup) Anaheim Produce Company (fresh fruit) and Hollywood Scoops (hand dipped ice cream) Warning: seating is entirely outside so this may not be the best option for the heat of the day.