When migrating data into the Nonprofit Success Pack (NPSP), the most efficient and robust tool is the NPSP Data Importer. This tool was specifically built to handle the 'multi-object' nature of nonprofit data in a single pass.

Why NPSP Data Importer is the Correct Choice:

Single File Processing: Standard Salesforce tools like Data Loader or the Data Import Wizard usually require separate files and separate 'upserts' for each object. You would have to import Accounts, then export the IDs, then import Contacts, then export those IDs, then import Opportunities.

Complex Logic in One Row: The NPSP Data Importer allows you to put the Contact Name, the Household address, and the Donation Amount on the same row of a CSV file.

Automatic Relationship Management: When you process the file, the Importer automatically:

Creates or finds the Contact.

Automatically creates the Household Account.

Creates the Opportunity (Donation) and links it to the Account.

Creates the Payment record (if payment info is included).

Handles GAU Allocations and Campaign Memberships.

Dry Run Capability: It includes a 'Dry Run' feature that validates the data and checks for duplicates before any actual records are created, which is vital for a clean migration.

While Workbench and Data Loader are powerful technical tools, they lack the specific NPSP business logic that makes the NPSP Data Importer the fastest and safest way to move a legacy database into Salesforce using a single flat file.