My dentist in Belize City...
Dr. Arden Wayne Usher
2529 Central American Blvd at Mahogany Street
Phone: 227-4999
No complaints.

Not sure why you asked about Chetumal, but if you're looking for a good dentist in northern Belize...
Dr. Garcia, 56 5th Ave, Corozal
Phone: 422-2355