What do you mean by best? Most cellphone stores are authorized retailers; meaning that they don’t set the prices for the plans but the company does i.e AT&T. In my experience Sprint is the cheapest followed by AT&T and then Verizon. Verizon is usually the most expensive but they have the best coverage (mostly). A lot of customers in Brooklyn say AT&T always drops calls here.
Sprint has a family plan starting at $69.99 (excluding taxes and fees of course)
You could look up all the plans and info on their websites.