To create 500 Accounts and ensure there are no duplicates for existing Account Sites, the best option is the Data Import Wizard. The Data Import Wizard in Salesforce is designed to handle bulk data import while also offering built-in duplicate management features. This makes it the optimal choice for importing a large number of records and preventing duplicates, compared to other tools such as Sandbox templates, Data Loader, or Salesforce-to-Salesforce, which do not specifically offer the same level of control over duplicate management during import.