I've always felt that L.A., Chicago, New York, Las Vegas, Philly, Pittsburgh, Boston, parts of Miami, PHOENIX (by far!), and DENVER were very, very dangerous places.
I would not be caught dead in Denver under any circumstance, nor Phoenix. NEVER.
The most dangerous city worldwide has got to be Jerusalem, ugh, I'd never ever want to go there.
In fact pretty much every city in Africa and the middle east is dangerous, I'd stay away from it period.