IMO, without a doubt it's the hand-made ice cream cookie sandwiches!
I prefer to get them at Sleepy Hollow in Liberty Square in the Magic Kingdom (as you have the best chance of getting fresh-from-the-oven cookies there). It's a small window-service place on your right just after you cross the bridge from the Hub.
In addition to Sleepy Hollow, you can also get them at:
-Main Street Bakery in the MK
-Fountainview Ice Cream in Epcot (Innoventions West, facing the big fountain)
-Dino Bite Snacks in Dinoland at Animal Kingdom (on the outside of Restaurantosaurus, near the DVC kiosk).
-I've also heard that they're available in the food courts of the Value resorts, but cannot confirm this. (At least until I'm there in September)
On Main Street and in Dinoland you have the option of chocolate chip, oatmeal raisin and sugar cookies. I think Sleepy Hollow it's chocolate chip only, and I don't know about Epcot.
-Rob