God the list could go on forever but I'll keep it short: Sandnes, Norway. Infamous across Norway for "Ruten", the bus station, which is a no go area. Drugs dealt 24/7 and repeated rapes. And I had the unfortunate pleasure of living about 10 mins away by bus.
Kosice, Slovakia: Half of it is literally a slum
Newquay, England: Full of lads trying to get their hole in caravans. Fight central.
Edit: I second Barrow and Tegel Airport. Mind you Schoenefeld isn't much better.