Abstract Nitrogen-vacancy (NV-) centres in diamond offer an excellent platform for device design and development in the emerging fields of quantum sensing and quantum network, but fabrication of quantum-grade diamonds with highly coherent NV- centres needed for advanced applications presents a pressing challenge. Here we report a major advance in generating NV- centres in diamonds via tailored high-pressure and high-temperature (HPHT) synthesis and post-grown annealing treatment. The resulting well dispersed single NV- centres in type-IIa diamonds exhibit long spin coherence times reaching the highest levels achieved in diamonds prepared by more elaborate and restrictive chemical vapor deposition techniques. Moreover, we produce high-density NV- ensembles in <100>-grown type-Ib diamonds with superb zero-phonon lines considerably sharper than those of native NV- centres in as-grown diamonds. These findings demonstrate a superior synthesis and optimization protocol for creating top-quality NV- centres to meet the most stringent requirements for a wide range of emerging quantum technologies.