I'm not from the USA so here people don't know much about it.
I'd say the most overrated is NYC, because everyone thinks it's cool, there's shopping, art etc., but I'd never live there considering the noise and the shitty snow and snowstorms, the greyness of the city etc.
Watching CSI NY has confirmed that I wouldn't like it.
The rest of the US is all underrated, except california and miami.
Everyone here on the forums says cali is a shithole, but from abroad you just see the nice beaches, the nice weather, combined with the money.
In europe the rich cities with technological companies are in places with low sun hours per year (think germany), the mediterranean isn't anything like that, the souther you go, the poorer the people are (except in the french riviera and other special places for rich people), the shittier the roads, hospitals, economy. If you're rich it's okay but most people can't buy 2 luxury houses or live off their money without working a lot (because money is useless if you don't have the time to use it).
In cali you have the mediterranean climate and the silicon valley in the same place, it's like a dream. Also as a swiss I'd be used to high cost of living.
Miami looks nice but from what I heard it's got southern mentality so it's not really compatible for european people I guess?
Also alligators and swamps.