I was never very impressed with any of them (I have eaten at Pecos Bills, Cosmic Rays, Columbia Harbor House, Casey's). That is until my last trip when Comsic Ray pulled ahead of the pack. I can only eat so many hamburgers before I can't stand the sight of one, so I was so pleased to realize that CR sells rotissarie chicken with actual vegetables. Not a fry in sight. Plus I love sitting on the patio overlooking the castle. I feel like I'm not missing anything.
Columbia Harbor House for clam chowder and 1/2 sandwich. I like the fact that you can go upstairs and get away from the crowds.
El Pirata y El Perico is my favorite, unfortunately, it is open seasonally and usually not when I am at WDW.