Definitely those areas that are defined as slums and/or ghettos. Living in Cleveland, I have seen many of these. But the ones in Cleveland aren't what really put me on edge... the "ghettos" here, although rundown, poor, and mostly minority-based, (i.e. fitting the stereotype of a slum or ghetto), are among the safest such areas in the nation. It's high crime, violent areas in general that worry me... the ones found in L.A., D.C., Chicago, Atlanta, and to a lesser extent, in Cleveland.
Having spent a month in Germany over the summer (in the Munich vicinity studying abroad actually), I wondered why the US can't be more like Germany. The city of Munich has hardly a slum, and it seems everybody had money to feed and shelter their families. Of course, as a friend of mine there explained, unemployment packages are very generous - so generous that many Germans find it easier to be unemployed than to seek work!!! In addition, Munich is the richest city in Germany, and one of the top five richest in Europe. Now Berlin has quite a few slum areas (at least relatively).