My personal favorites are:
MK - Le Fou's Brew (I know it is a drink but I am addicted), Dole Whip Float, popcorn, ice cream sandwich from Main Street ice cream parlor (love getting it with mint chocolate chip ice cream), swirl cone from Auntie's Gravity's, citrus swirl from Sunshine Tree Terrace (just tried this in January, and now it is a must have for me).
Epcot - popcorn, creme horn from Norway bakery, caramel corn from Karmel Kutch (again I first had this in January, and now I must have it each time I go), frozen coke from Club Cool
DHS - Chocolate/peanut butter cupcake from Starring Rolls, popcorn, Carrot Cake Cookie sandwich, frozen lemonade (not the minute maid frozen lemonade but the one that is more like an icy)
AK - popcorn (notice the trend
), swirl ice cream cone from Anandapur Ice Cream Truck