MK: Columbia Harbor House. Im from New England, good place to get some northern comfort food. When the upstairs rooms are open, great atmosphere with all the 18th/19th century nautical theming. I would say Pecos Bills, but I haven't been there since they made it all Tex Mex. The reviews are good though.
EP: Cantina de San Angel, some of the best quick Mexican food I've ever had. Right on the water in the World showcase lagoon. Katsura Grill, probably one of the best quick serve values. Portions are great, food is good, prices are reasonable and the dining room has a great view of the Japan pavilion. Also sunshine Seasons has a lot of great options, something for everyone and the deserts are out of this world.
HS: Backlot express is really the only place I eat there. Typical burger and sandwich fare, but theres a lot of cool movie memoriblia on the walls. Take a look around that you're bound to find something interesting.
AK: Yak and Yeti quick serve has great Asian inspired cuisine. Best way to describe is good Chinese takeout food. Also Flame Tree Barbecue. Good sandwiches, excellent ribs. I will say this though for both in AK, seating is outside, and AK being a generally hotter park sometimes doesn't do the food any favors by having to eat it outside.
Honorable mentions: the fish and chip stand in the UK pavilion in epcot has some of the best fish and chips ive ever eaten. Its my wife's favorite and a must do stop every time we go. Grab a basket and a quick pint from the beer stand and you cant go wrong. Cosmic Rays in MK is a solid choice when travelling with families.