For a data migration involving tens of millions of records, which method is most efficient?

The Bulk API is specifically designed for loading large volumes of data into Salesforce efficiently. It employs parallel processing, which allows multiple records to be processed simultaneously, significantly speeding up the data migration process. This makes it particularly suitable for scenarios involving tens of millions of records, as it can handle larger batches and is optimized for high-volume data load.

While other methods, like the SOAP API, NPSP Data Import, and Data Import Wizard, can be used for data migration, they are not as efficient for such high volume use cases. The SOAP API is better suited for smaller, more transactional data operations, while the NPSP Data Import and Data Import Wizard are generally user-friendly tools for typical, smaller import tasks, each with their own limitations on the number of records they can efficiently process at one time. Hence, for a data migration involving a significant amount of data, the Bulk API is the most effective method.