I love Casey's Corner. Hot dogs, baseball, and a view of the Castle. Can't beat that. They should sell roasted peanuts (but I imagine they don't want to deal w/ the leftover shells).
I tend to think of the Brown Derby as underrated, only because I don't seem to read a lot about it when the subject of favorite restaurants comes up on the various Disney message boards. It's a classy restaurant, couldn't be more appropriate in contributing to the overall theming of the Studios as it is an exact internal replica of the original Brown Derby of Hollywood, and the menu includes the Cobb Salad which was invented by the BD's owner and popularized by Sid Grauman himself. I've never tried anything else there but the Cobb, and it is fantastic.
Here you leave today and enter the world of yesterday, tomorrow, and fantasy.