In the Magic Kingdom our favorite was Pecos Bill. The burgers and taco salad were delicious and their toppings bar can't be beat. We also enjoyed Casey's hot dogs and we liked Cosmic Ray's for the large selection.
At the Animal Kingdom we liked Flame Tree BBQ, but my personal favorite was the Yak & Yeti CS. The food was exactly what you would get at the table service, the portions were large and the price was good. I highly recommend it. The pizza was good at Pizzafari, but nothing that you couldn't find elsewhere in the park.
At Epcot our two favorites were Sunshine Seasons (the selection is terrific and I really don't think there is a bad choice to be made there) and Cantina de san Angel in Mexico. If you eat in Mexico, be sure to try the Conga juice - it is delicious and so refreshing!
I can't really comment on the Studios, since we only had a quick bite at the Studio Catering Company, but their greek salad was very good and a nice change from the regular park food.