For the marketing associate at Cloud Kicks importing a list of contacts into a data extension with attributes like ContactID, MobileNumber, and EmailAddress, the best choice for the primary key would be ContactID. The primary key in a data extension is used to ensure uniqueness of each record, preventing duplicate entries. Given that ContactID is described as the unique identifier for each contact, it naturally serves as the most appropriate choice for the primary key.

Using ContactID as the primary key guarantees that each contact will be represented only once in the data extension, regardless of whether they might have multiple email addresses or mobile numbers over time. This approach aligns with best practices in database management and data integrity.