Kind of a tough call. Here's one from each park.
MK - Cosmic Rays. They serve chicken, burgers (including veggie), soup, salads and some other sandwiches. The best part is a wonderful topping bar with raw and cooked onions, sauteed mushrooms, lettuce, pickles, various sauces and lots more. I get a veggie burger and pile it high with toppings.
EC - The Land Food Court. They have a wide variety of foods. Always something for everyone in our party.
MGM - Rosie's. Also good veggie burgers though no toppings bar. Also, hamburgers, hot dogs, pizza and perhaps some other stuff.
AK - Tusker House. This place probably has the best food of any counter service. Roasted chicken platters, grilled vegetables and more. More upscale than the average counter service.