What is an expert dentist called?

Doctor.

They should have a doctorate in dentistry. In addition to general dentistry, there are about 9 recognized dental specialties in the US, Canada, and Australia. To become a specialist requires one to train in a residency or advanced graduate training program. Once residency is completed, the doctor is granted a certificate of specialty training. Many specialty programs have optional or required advanced degrees such as a masters degree: (MS, MSc, MDS, MSD, MDSc, MMSc, or MDent), doctoral degree: (DClinDent, DMSc, or PhD), or medical degree: (MD/MBBS specific to Maxillofacial Surgery and sometimes Oral Medicine)