Avoid the churros! We end up always getting them at least once during our trips to WDW, but often times they disappoint. Lately, the ones we have gotten have been a bit stale.
At Magic Kingdom, my favorite is definitely the cheeseburger spring roll at the cart at the entrance to Adventureland! My daughter loves the Cheshire cat tail at Cheshire Cafe (basically a twisted croissant with chocolate chips inside with purple and pink icing piping on top). It's by the tea cups and across from Cosmic Rays.
At AK, I like the Pongo Lumpia (crispy egg roll/spring roll with pineapple and cream cheese filling) at Pongu Pongu that you find near the exit of Flight of Passage ride in Pandora.
At Epcot, we like a few things - caramel popcorn at Karamell-Küche in the Germany pavilion, Kakigori (shaved ice, aka Japanese snow cone) at Kabuki Cafe in the Japan pavilion, and the Macaron Ice Cream Sandwich at L’Artisan des Glaces in France.
Finally, WDW loves a special occasion cupcake, which I personally love too! So keep an eye out at the various parks...