Fast answer
For a first Valle d'Itria stay, choose the base by the part of the trip that must feel easy. Locorotondo or Cisternino fit quiet white-town evenings, Martina Franca fits a fuller baroque night, Alberobello and the countryside fit trulli atmosphere, and masserie or contrade fit travelers who accept a car-led rhythm. If beach time, rail convenience, or sea-facing dinners are the fixed point, compare Monopoli before committing all nights inland.
If you only do one thing
Default to a walkable white-town or Martina Franca stay when evenings on foot matter. Choose countryside trulli or a masseria only when parking, dinner transport, and daily drives are already part of the plan.