That generaly depends on which gharana you are learning or wish to learn. Dehli and Calcutta are good places but quite expensive to stay for a long time. I personaly learn from a teacher in Varanasi. the cost of living there is very cheap and there are many good teachers. However, there are many bad teachers as well. So you have to be careful. If a teacher is connected to a shop then they are not good. They are merely sales people to get you to but some tabla. Also, Varanasi is a very nice city to live in. Any ideas which gharana you like?