Worst is such a subjective term. Places that I wouldn't recommend because I thought they were dumps are places that people I know seek out because they love it there. Places I love for my own reasons are places people I know hate because they aren't looking for the same things.
For any given country the chances are there's a really nasty social housing project that's worth avoiding. Maybe the military installations with high security and the nuclear power stations aren't great tourist attractions either.