Ice cream from the France pavilion in Epcot. It's the best ice cream on property - hands down. Keeping with Epcot, I'll add in maple popcorn from the Canada pavillion and caramel corn from Karamell Kuche - though pretty much anything from there is going to be a winner.
Also, I'm going to second two of the above options. The Tropical Serenade was fantastic. Coconut Dole whip and POG juice, I mean what's not to love? It's the best Dole whip treat I've ever had. Even the cake pop was yummy, and I'm usually not a big fan of them.
The other one that I concur with is the cream cheese stuffed pretzel. We were in HS back in Dec and because of chasing our
Genie+ selections, we ended up going a bit past the point where we badly needed food and caffeine. After riding Rock and Roller Coaster we saw the cart selling those pretzels. We each bought a pretzel and a Coke, and then sat on a wall outside of the Brown Derby munching. Were we just so hungry and in need of a Coke boost and a place to sit? Maybe, but those pretzels were one of our favorite treats that trip. So good!
On the flip side - after hearing so much hype about the spring rolls in MK Adventureland we tried them last year and while they weren't terrible - they were the definition of "Meh". Very greasy, and way overpriced - even for Disney.