I've always found there to be a fairly wide variety at each location. Here are some of my personal faves, in no particular order:
Magic Kingdom:
* Columbia Harbour House (love the Starboard Deck Sandwich)
* Cosmic Ray's (good if you need lots of variety)
Epcot:
* Cantina de San Angel (the kid's plate is good and perfect as a small adult meal for when you're eating around the world)
* Sommerfest (I love bratwurst!!)
* Yorkshire County Fish Shop (I can't get enough of the fish & chips!)
MGM:
* I always have a hard time finding good stuff here. Usually I head to Beaches & Cream at the Beach Club resort on my MGM days.
Animal Kingdom
* Tusker House (had great variety before, not sure what the offerings will be once they make their changes)
* Flame Tree BBQ