For some reason, we always end up near Pecos Bills for an early lunch. Early, it's a nice quiet, cool place to get a burger and any "fixin'" you could think of.
My favorite tip I learned from these boards, Order a double burger meals and extra buns. You can add extra sides if you need to. On our last trip, DH, DD2, & Me - ordered a Double burger meal, extra bun, 2 large drinks and a kids meal. We had more than enough. And DH is a BIG eater! I know it sounds crazy to order an extra bun, but you are paying for it, they actually have a button on the register for an extra bun.
You can use the fixin's like chili and cheese to spruce up your fries!
If your looking for counter service and you find your tummy rumbling in Frontierland, go to Pecos Bills!
Cosmis Rays is good in Tomorrowland.
Columbia Harbour House in Liberty Square is good too!