What is the proper noun of doctor?

Doctor is a common noun. It can be capitalized when it is used as a title before a person's name, but it is not a proper noun.

For example:

"I went to see Dr. Smith today."

"Dr. Smith is a very good doctor."

In the first sentence, "Dr. Smith" is capitalized because it is used as a title before a person's name. In the second sentence, "doctor" is not capitalized because it is used as a common noun.