Above all I want London but if it's going to stay in the , I think the best and most relevant place would be the US-Mexico border, a place where GTA levels of violence are not unrealistic, if not:
New Orleans (Crazy street gangs, guns everywhere, unique culture and architecture, Voodoo, bayous.)
Houston (Dirty South gangsters, grills, purple drank, AKs and Mexican Cartels.)
Hawaii (Yakuza, Triads, machete wielding Native meth addicts, beautiful scenery)
Seattle (Rain, East Asian gangs, bikers)
New England (Perfect for an SA style map, Irish mob, Mafia)
Outside the US my top choice is Russia. I really, really want a GTA set in Russia, I think it's just such a totally insane place perfect for GTA.