Paris, without a doubt. A filthy, unsafe, overpriced overrated dump of a city.
Dubrovnik, sadly. A classic example of a beautiful, historic place which has been ruined and turned into a theme park by mass tourism.
I’m going to defend Cornwall, though. It’s a county I know well and it’s really only an overcrowded nightmare in July & August. The rest of the year, it’s lovely. Even Newquay & St Ives are tolerable.