In general, most letters to pastors should be addressed to "The Reverend John Smith," with the name of the specific pastor replacing "John Smith." A letter to a Roman Catholic priest is simply addressed to "Reverend" with the priest's name appended.
If the pastor you are writing to has a doctorate, address the letter to "The Reverend Doctor John Smith." No distinction for doctorates is made in addressing a letter to a Roman Catholic priest.
Address the pastor within the letter as "Dear Pastor." Address an Anglican or Episcopal priest as "Dear Father," and a Roman Catholic priest as "Dear Reverend."