The UK generally is underrated. London, in particular, is a place we all take for granted. But it really is one of the greatest cities in the world. I went to the London Marathon last year, and pretty much everyone I spoke to said the same thing: "why the hell don't I come to London more often?"
Cambridge would be my next choice. In fact, I'd nominate Cambridge as my favourite place in the world. I especially love it on bright, cold, sunny mornings in Autumn. I go and have a coffee, watch the students and professors drifting about, then spend the rest of the day looking at the beautiful buildings and mooching around the bookshops. It's incredible to think of the people who've studied there, and walked the same streets - Newton, Darwin, Wordsworth, Byron, Nabokov, Wittgenstein, Stephen Fry, Bertrand Russell, etc.
Manchester. People have this fixed idea of it as grim, scruffy and post-industrial. Actually, it's a place to watch. I wouldn't be surpised to see it booming ten or twenty years from now.
Northumberland is underrated. The whole area around the English-Scottish border is gorgeous (and relatively empty).
Norway is pleasant - quiet and civilized. I also like Brussels. Portugal is underrated, especially Lisbon. Oh, and Madrid. Everyone goes to Barcelona instead, but I really loved Madrid (also much bigger than I expected).
I guess it depends what you like. Lots of people have this thing about Australia, for example - like it's some kind of paradise. Well, fine, if you like hot sunshine and beaches I guess it is. But I'm not a beach person, and I really dislike the heat. Personally, I can't imagine anything more boring than living in a small Australian town.