The caramel squares covered in chocolate with sea salt sprinkled on top are my favorite. I like the milk chocolate ones, but they have dark chocolate ones as well.
As for the value for size question, the covered caramel squares I was talking about above are pretty thick (tall) and, while they fit comfortably in the palm of your hand, they are still a pretty formidable treat. They are really rich and I always feel fairly full after eating one. I get as much enjoyment out of having one of these as I do having a Dole Whip in the MK, and I probably feel more full after having the caramel. Your mileage may vary, though. Some of the other treats there, like the caramel (and other stuff) dipped pretzel sticks and the large dipped strawberries may seem more like they are worth the cost for what you are getting.
For my money, though, the mix of the sweet chocolate, the rich caramel, and the bite of the salt on top of the chocolate covered caramel squares is the best snack in all of Epcot.