Was in Medellin last October. Perfectly safe. Certain areas you don't really want to go wandering around alone at night, but that's true of every city.
I did end up lost in Comuna 13 for about an hour, but it was during the afternoon. I kept walking up sets of stairs that ended at different people's homes. It seems what was once one of the most dangerous places in the city, it now has walking tours in English.
Pretty sure Singapore have bad neighbourhoods too. By their standards. Their worse neighbourhood is probably better than most good areas in many cities.
>> Specific areas – Geylang and lower-cost government housing – suffer from more serious crimes (muggings, loan sharking, and illicit drug use) although not on the scale of a similar sized U.S. city.
