For me, the best in-park ones are Tusker House (AK), Biergarten (EP), and Garden Grill (EP).
Tusker House has a great breakfast with cool African-inspired dishes (corned-beef hash, beef bobotie, some thing with yams and coconut that is amazing), but also the typical eggs, bacon, etc. It is a character breakfast with Safari Donald and friends.
Biergarten is a German restaurant with communal seating, so you will be mixed with other parties at a table. It has a fun atmosphere with a polka band. The food is all sausages and other heavy meat, pickles, spaetzle, and some other interesting items. The apple strudel is awesome! This is a good one for any guys who are big eaters in your group.
Garden Grill is different in that you are seated and get a salad, then a skillet with hearty meat and potatoes (I want to say there is sausage, pot-roast, mashed potatoes, some green beans, carrots, etc. - a good country meal) and also french fires, and mac & cheese. The skillet, fries, and mac are all unlimited and you can ask for more and they will bring it. There is a desert as well after that, but it is not unlimited. This is a rotating restaurant in The Land pavilion that overlooks the sets from Living With the Land. It is also a character meal with Chip & Dale, and Farmer Mickey.
If you are looking to seriously eat, these places will do the job!