Magic kingdom - don't love the QS options, but there are TONS of great snacks. Pineapple Dole whip floats, Mickey waffles from sleepy hollow, Cheshire cat tail, smoothies from Auntie Gravity's, etc
Epcot - EVERYTHING IS DELICIOUS. Most countries in the world showcase have quick service options, but my favorites would be Japan's Katsura Grill (which is kind of hidden behind the Japanese gardens, so it feels like an escape from the parks), and Norway's Kringla Bakeri (which has some really yummy sandwiches, in addition to amazing pastries). In future world, Sunshine Seasons is a food court, which is great if you have a group of picky eaters, but I've found the food hit or miss. For snacks, I could probably go on all day, but there's really nowhere to go wrong. I love the poutine (although I think it's listed as cheese fries) from Refreshment Port, which is near Canada. There are great pastries in France and Norway. Germany's pretzel bread and bratwurst are excellent, if you like German food. Oh, and so so many delicious caramel candies and treats at Karamell Kuche. And if you're there during any festival, there will be dozens of options for
DDP snacks at all of the food booths. Always good.
Hollywood studios - For QS, I haven't tried it yet, but heard good things about the sandwiches at Woody's Lunch Box. I haven't come across any must-have snacks at HS either, but they have some great alcoholic drinks like craft beer and cocktails (not for DDP snacks, but could be included with a QS meal), if that's on your radar.
Animal Kingdom - Satuli and Flame Tree are our favorites for QS. Both have huge entrees, lots of options, great seating areas. There are also some snack options from Flame Tree BBQ that are great. In Pandora, the Night Blossom drink from Pongu Pongu is a must-have snack for us, but it's big and very sweet, so we usually split them.
If you're a Starbucks person, there is one in every park, and ALL of the drinks (plus most of the food) count as snack credits, so it's a great option if you want to get a giant, 6 espresso shots, venti fancy latte.
Just please, please, don't waste your credits on things like bottled drinks or a piece of fruit! Pay out of pocket for stuff like that, and use your snack credits like small meals and special treats.