I wouldn't even bother picking a specific item and just say anything in Karmell Kuche is worth it.
In Magic Kingdom I like the cinnamon roll at Gaston's. You could also split a hot dog at Casey's if they have an interesting special and get custom popcorn at Main Street Bakery.
I don't have any good recs for permanent location snacks in Epcot, but if you're there during a festival, I always like the Honey booth between Canada and Future World. India, China, and Ireland also tend to have great booths, especially the chocolate dessert from Ireland.
In Studios I almost always get a ronto wrap. Not a fan of the breakfast or veggie ones, just the original. I also like the loaded tots and fake poptarts at Woody's lunchbox.
Animal Kingdom has Tamu Tamu and Pongu Pongu, which both serve special dole whips and frozen drinks.
Thinking about this I realized a lot of my favorite snacks are just alcoholic drinks, so maybe not the best for DD19, but if you want some then I really like the margaritas at Cava del Tequilla, anything from the Brown Derby Lounge, and the slushes at les vins des chefs du France.