valued customer; prime client; good customer
Used in business contexts to refer to a particularly important or loyal customer, often one who brings significant business.
That person is a valued customer of our shop.
We provide special services to our prime customers.
お得意様 is a polite, honorific way to refer to a customer, often used in direct address or formal contexts, whereas 上得意 is a more objective classification of a customer's importance.
Compound of 上 (じょう, 'upper, superior') and 得意 (とくい, 'customer, client'), literally meaning 'superior customer'.