The best neighborhood is a category that, besides the fact that people can always argue about what they LIKE, has its own "rules". What makes the best neighborhood? It has to be clean, quiet, in a good location (near of important places) and stuff like that. In my opinion, Piantini is a nice place, with beautiful, expensive buildings. But since its in the center of the city (the "poligono central"), it has the worse traffic of the D. N. Just think of Churchill, Kennedy, Lincoln and 27, every morning at 7:30-8:30 and 5:30-7:30 at noon, and you'll agree with me.
Arroyo Hondo is, THE neighborhood for rich people in the DR. It has that fame. It has to be the place with the highest income per cap in this country. Or at least it was before Cacicazgos came along. AH hasn't too many buildings, but has most of the biggest houses around here. But as someone said here before me, the condition of the streets/infrastructure out there is poor. Some of the areas are pretty much isolated(?) and it can be pretty scary to drive, dont even mention walk, at night. Besides, it has not many commerce areas.
Gazcue is a memory from the past. It was the neighborhood of wealthy people during the tirany of Trujillo. Has some of the best architechture of the city, with many trees and pedestrian-friendly streets. But its a boring place to live. Its also partialy unsafe.
Now enter Cacicazgos. It's a breathtaking place to be. It has some of the biggest, most beautiful houses of the city, along with many of the coolest residencial buildings of the area. But it's for two special things the cacicazgos is, for me, the nicest place to be: The Mirador and the Anacaona Ave. I mean, just driving everyday trhough the Anacaona is an experience itself. The view of the mirador at one hand, and a batch of coolest buildings at the other, with most of the embassies located here, its an exclusive experience in Santo Domingo. Cacicazgos is minutes away from SD Country Club, the herrera intnl airport, bella vista mall, and places like that. It is quiet, clean, safe, near of everything, but not so near that you have to face heavy traffic to get in or out, and stuff like that.