The thing about desserts is that they change periodically - esp for things like fruit desserts (withthe seasons). Having said that, here's what I recall:
50's Prime Time - best was the kids' make your own sundae
Kona Cafe - chocolate lava cake (or what ever it's called there) and the kids' sundae with the face and the cotton candy clouds. They used to have a whole ton of choices there, but now only have a few....
I haven't tried the other two yet. The best dessert we ever had at WDW was the chocolate lava cake they once had at the Flying Fish. We had it twice, it was their signature dessert, but when we went back a third time, it was no longer there. The Kringra la bakery, the France bakery, and the treat shops in MK seem to stay about the same, but many of the TS places change desserts.
The one I didn't get to try was the individual wedding cakes at GF, or the crystal Slipper (CRT, during "Happiest Celebration," I think)