Before reading this thread, I had never stopped to think that I do almost all my snacking at Epcot and MK! I had to really think to come up with things I'd tried at AK and DHS.
The best (and only) snack I've tried at DHS is the Butterfinger cupcake. It lasted me 3 days because it was all I could do to eat one single VERY sweet bite at a time. Thank goodness for the room fridge!
The best (and only) snack I've tried at AK is tea from the Royal Anandapur Tea Company, which I highly recommend. We almost always travel during the coldest times of the year, and the hot chai latte is the perfect thing to warm you up.