I can't say anywhere ever really disappointed me. I like to do my research before going somewhere. However, in terms of surprises- I was absolutely blown away by Medellín, Colombia. It is not underrated as so many have fallen inlove with this city- but it still has to be one of my best destinations.
If you'd like underrated though, I'd have to say this little island in the Philippines called Apo Island. Amazing dive spot and warm, friendly people.