Cities are good for sustaining a community of dancers, but quality of coaching really seems to be an individual thing - it doesn't matter where you study, it matters that you have access to someone who can really help you. If you define the best coaches as those who have students at world- or blackpool-finalist level, it's a pretty short list, and so necessarily a handfull of home locations. If you can't study with someone like that all the time, then I think the next best thing is to work with them a few times a year, and then seek out a more available coach who can best (compatibly) assist your development between those infrequent appointments. But there's more to dancing that just coaches, so in the end, the best locations are probably those that have a suitable everyday coach or two, but are also places where you can have a partner, and a non-dancing life.