The best practice for creating a subscriber key in Salesforce Marketing Cloud is to use a unique, persistent value like a Customer ID. The subscriber key serves as the unique identifier for each subscriber, so it should not change over time, which makes a Customer ID ideal as it is often assigned at the start of a customer relationship and remains consistent.

Why Not Use Email Address or Phone Number: These can change over time and are not as stable. Using a Customer ID helps maintain continuity even if a customer updates their email address or phone number.

Salesforce Documentation Reference: For more information, refer to the Salesforce Marketing Cloud Guide on Subscriber Keys.