Amazon S3 Glacier Deep Archive is the most cost-effective storage service for long-term data archiving where data is rarely accessed and retrieval times of up to 12 hours are acceptable. S3 Glacier and S3 Intelligent-Tiering are better suited for data that may need to be accessed more frequently, and Amazon S3 Standard is optimized for frequently accessed data, hence, it is not cost-effective for long-term archival needs.