For me, the dole whip float (with pineapple orange swirl) is a must.
It's impossible to pick one favorite in Epcot. The salted caramel ice cream in France is amazing. Also anything from the bakery in France - sweet or savory.
Italy has Lemoncello, which may not be a snack, but it is the best booze bargain in WDW.
There's an artisanal cheese plate in Germany that is good. I would get it again and again for the ham that was on it. The caramel corn is great, too.
The Kakigori is pretty good, but only with the sweet milk - otherwise it's just a snow cone.
Everything in the bakery in Norway is good. I like the cloudberry cream horn or the rullekake.
While not technically a snack, the fish and chips in the UK always hit the spot.