Well, for
Magic Kingdom, I consider the Main St. Bakery to be somewhat of a hidden gem for lunch. It's just basic sandwiches (turkey, ham), but the bread is so fresh, great quality meats, great size, and you can polish it off with one of their yummy bakery treats (like their homemade ice cream cookie sandwiches!). We loved this place and we are going to make it a regular lunch place for us for years to come. It's so fun to eat on Main St. and if you go around noontime, you will catch the Main St. Philharmonic.
If you like hot dogs, you might try Casey's Corner, also on Main St. I don't eat hot dogs personally, but I've read great reviews about this place. and they have a live pianist! There's just something about dining on Main St....
Can't help ya much with
Epcot & DHS, as I've only been to the places you mentioned, but I bought a drink at the Electric Umbrella when we were there and thought their food looked pretty good from what I saw.
At DHS, I've heard pretty good reviews about Starring Rolls & also Pizza Planet.
DAK: We ate at Pizzafari. Pizza was just generic WDW pizza, but the italian sandwich was great. If you are interested in venturing over to AKL for lunch, the Mara is fantastic & the animals at AKL are fun to visit before or after lunch.
The only other CS place we went to that is definately worth mentioning is Captain Cook's at the Poly. GREAT sandwiches and they have self serve Dole Whip machines!
HTH!