Santiago
All three are very warm in the summer.
Bonao is more humid since it sits in one of the rainiest parts of the country. The water table there is also very high, lending to added humidity.
Moca gets more rain than Santiago, less than Bonao. Heat-wise, it is about the same.
Santiago generally enjoys a nice breeze from the east.
The area to the north of Moca, Jamao del Norte, is, indeed, much cooler. The last time I drove that road, I saw several properties up on the top that were for sale.
People are now building to the north of Santiago , in the foothills of the Cordillera Septemtrional. One such project is called, really: "Jacagua Hills".
If it is cool you want take Jarabacoa, just because it is closer to everything. #2 would be up in Jamao, with less service, more distance but cool.
HB