What term describes a sponsor or benefactor, especially a regular customer?

The term that best describes a sponsor or benefactor, particularly one who is often a regular customer, is "patron." A patron usually refers to someone who supports an organization or individual, often in the arts or service sector, through financial contributions or consistent business. In many contexts, a patron not only provides resources but also engages with the entity they support, making them a regular customer or client.

While terms like "investor" and "benefactor" may imply some level of support, they often do not capture the idea of a consistent and personal engagement with a business or service, which is fundamental to the definition of a patron. Similarly, while "client" refers to a customer receiving services, it does not inherently convey the idea of support or sponsorship associated with the term "patron." Thus, "patron" is the most fitting choice for describing someone who regularly supports a business or cause.