We have a few "must-do" restaurants at MK - Cinderella's Royal Table (dinner), Crystal Palace (lunch), & Liberty Tree Tavern (lunch).
At Crystal Palace, I love the corn spoon bread & the Mandarin orange salad dressing. At Liberty Tree Tavern, I have to get the Ooey Gooey Toffee Cake - my fav dessert in all of WDW!
EPCOT is harder. Every year, we try a new World Showcase restaurant, &, while they're all good, we haven't found a favorite yet. Right now, Garden Grill in Future World is a must-do character meal for us.
One thing I have to get while I'm in EPCOT is a strawberry tart from the French bakery.
In DHS, our only "must-do" restaurant is Sci-Fi - not really anything there, though, that I *have* to get.
In AK, we like Yak & Yeti - DD has to get the pot stickers, & we all really enjoy the pineapple wonton dessert! I can't eat MSG, & the chef always makes me a really good beef & broccoli dish.