By South does that include the Southwest? If so I would vote for Phoenix. It is one of the largest hubs for the cartels and it is one of the largest wholesale drug markets in the USA and I would argue the world.

You have the cartels there, traditional African American and Hispanic Gangs, Outlaw motorcycle clubs, Asian Gangs, the Aryan Brotherhood, Eurasian OC, and some would say even some residual LCN activity (I do not think there is any personally) as Louis Caruso a Los Angeles LCN capo has an air condition business there as well as some Chicago associates. Basically every major OC group currently has or has had representatives there. Just my two cents.