Honestly, it depends where you go and what you order. Some places even serve McDonald's food, so watch out! I think Cosmic Rays is pretty good, I've had the corn dog nuggets, grilled chicken sandwich, and the cheese steak sub from there. The cheese steak is the only thing I wouldn't order again. It lacked flavor, and I had to put lots of ketchup on it. Another good choice in the MK is Colombia Harbour House for the fried fish strips. They also serve clam chowder in a bread bowl, which I have not tried. If you like hot dogs, Casey's Corner is the place to go.
As far as other parks, Tusker House in Animal Kingdom serves great food, far superior to McDonald's-type food.
MGM is probably the worst, IMHO, as far as fast food is concerned. I typically get the fish and chips from ABC Commissary, but they are nothing special.
Epcot has many excellent choices, including the Sunshine Season Food Fair in the Land. Many different choices. I particularly enjoyed the tuna sandwich and the barbecued chicken. In World Showcase you can get many different types of quick service food, which are nothing like McDonald's. You can get food from Mexico, UK, Norway, Japan, China, etc.
Hope this helps!