Parts of Miami right up there for me, really deprived areas.

Went to Watt's in LA which has a bad reputation, but it wasn't too bad, but this was the middle of the day and there were a number of burnt out cars dotted around the metro station - so I imagine it was probably a little more intimidating when the California sun went down... considering the area and the history was somewhat surprised Cardiff Uni sent a small group of us into the area, but nonetheless some good street art and Watts towers, so not a totally redundant visit.

Not in the UK or US but Paris is by far the roughest place I've ever been, and the only city I've been too which I felt truly unsafe in.