Anecdotally, I would say MetroWest. Just considering towns like Weston and Dover are among the wealthiest in the country. Then you still have Wellesley, Concord, Wayland, Carlisle, Natick, Newton, Sudbury; I could keep going. There are example of extreme wealth in both the south and north shore too (I'd argue north shore is prob #2), but MA as a whole is just one of the wealthiest states in the country.