The most dangerous city for which statistics are available is probably Ciudad Juarez. From wikipedia:
"The city is also the site of widespread poverty and violence, including an infamous series of unsolved murders of female factory workers.[23] The violence generated by the drug war translated into more than 2,600 killings in 2008. More than 1,600 of them occurred in Juárez, three times more than the most murderous city in the United States.[24] And that number of killings increased to 2,600 in 2009.[25] In 2010, 3,075 homicides took place in Juárez. This has led to a homicide rate of 229 killings per 100,000 inhabitants.[26] In response, business groups in Juárez have called for UN intervention.[27][28]
There were 1,400 murders in Ciudad Juárez in 2008.[29] In 2010, the number of homicides in the city reached 3,075.[30] The population of Ciudad Juárez have had to change their daily routine and many try to stay home in the evening hours. Public life is limited by fear of kidnapping or random violence. In February 2009, the U.S. State Department announced in an updated travel alert that "Mexican authorities report that more than 1,800 people have been killed in the city since January 2008." [31] On 12 March 2009, police found "at least seven" partially buried bodies in the outskirts of the city, close to the US-Mexican border. Five severed heads were discovered in ice boxes, along with notes to rivals in the drug wars. Beheadings, attacks on police, and shootings are common in some regions.[32]
In September 2009, eighteen patients at a drug rehabilitation clinic called El Aliviane were massacred in a turf battle; the victims were lined up in a corridor and gunned down.[33] The authorities had no immediate suspects or information on the victims.
Plagued by corruption and the assassination of many of its officers, the government is struggling to maintain Ciudad Juárez's police force, while other officers have quit the force out of fear of being targeted.[34] In late 2008, one murder victim was found near a school hanging from a fence with a pig's mask on his face, and another one was found beheaded hanging from a bridge in one of the city's busier streets.[35]
Between February 17th–19th 2011, 53 people were killed, including four police officers. State attorney general's office spokesman Arturo Sandoval was quoted as saying, "This is the worst violence we've seen this year." The increase in violence left city morgues overwhelmed, causing trouble for storing bodies. As of February 20th, 2011, Juárez averages eight homicides per day"