In Quito, these have been suggested to me (have not used them myself):
One name that comes up often is Dr. Montalvo: in
Quito: 3980147; cell: 0988646915, and email: montalvodentalcare@hotmail.com
Another is Dr. Ruben Barona, Office Phone (593-2) 227-7450 or cell: 0999244881. His address is Checoeslovaquia E9-72 y Suiza, Quito, Ecuador and his email is barona.viteri@gmail.com
http://plus.google.com/photos/108089047953129234429/albums/5537053759589354545
http://www.global-dental.net/
And for more expensive, cosmetic dentistry: http://www.kevadental.com
Two more English-speaking dentists in Quito are: Dr. Luis Flores Gomez (Av. Mariana de Jesús Calle A, Tel: 593-2-246-2651, E-mail: luflores@accessinter.net) and Dr. Diego Arcos Bronenberg (República de El Salvador 525, Edif. Rosania, Tel: 593-2-245-7268).
I’ll let Edd comment on Cuenca.