There are not enough sit-down restaurants at the parks in my opinion. I mean counter service is great and all, but every now and then when I am at Magic Kingdom, I would just love for someone to bring me my food! Animal Kingdom needs one too. Of course, Epcot makes up for shortages at others, and MGM has their share too. Here are my favorites...
Magic Kingdom: Pecos Bill's (Counter Service) or just grab a turkey leg and a frozen lemonade and sit on a bench
Animal Kingdom: Tusker House (Counter service) or Flame Trees (Best ribs on the planet)
Epcot: So many choices, but my current favorites are San Angel Inn (Pollo A Las Rajas for Lunch and Filete Motuleno for Dinner) or Le Cellier (everything is good here, especially the bread!) Also, Biergarten is a must for me every October (but I love the food, and I harass the staff by speaking German to them, poorly)
And my knowledge of places at MGM is limited, with it being a short boat ride (or long walk) from Epcot, I have always left MGM and gone to Epcot for food
And then when I really feel like splurging... I leave Disney all together and go to the Mall at Millennia and have dinner at the Cheesecake Factory.