A bit like plucking the random countries for population density, we could always pick a few cities that are indeed horrific.
(And as for those picked, Paris has a deeply seedy underbelly btw, really not sure I like it! Barcelona as far as I could see was split into good tourist vs slight shantiness. Can't speak for Seoul, but I did know people who couldn't wait to escape because living in that way was driving them insane! OK, this government sent them back because they only had a temporary work contract for three years...)