El Cibao is the best, by far!
Being from La Vega, smack in the middle of the El Cibao area which encompasses most of the middle of the country including La Vega, Moca, Santiago, Puerto Plata all the way to Nagua, etc., I am prejudiced for my area. Some of the spots already mentioned above including Jarabacoa and Jamao del Norte are very nice and the general area is industrious, varied from tranquil mountain towns to vibrating beach towns like Cabarete or Sosua.
Santiago is very nice and La Vega, at least during carnival time, is great and a nice quiet, civilized, good-for-raising-a-family town the rest of the time.
Santo Domingo, of course, is a very cosmopolitan city where you have access to just about anything and right this moment I wish I was at my neighborhood's El Nacional supermarket getting my weekly fix of 'arroz con leche' (rice pudding), majarete (sweet corn pudding) and 'habichuelas con dulce' (sweet bean soup--chock full of sugar, sweet potatoes and raisins).
Happiness for me goes through my stomach, most of the time.