Which Amazon S3 storage class is the most cost-effective when access patterns are unknown?

S3 Intelligent-Tiering is the most cost-effective option when access patterns are unknown because it automatically moves objects between two access tiers (frequent and infrequent) based on changing access patterns. This optimizes storage costs without performance impact or operational overhead. S3 Standard is designed for frequently accessed data and would be more expensive if data becomes infrequently accessed. S3 Standard-IA and S3 One Zone-IA are for infrequently accessed data and would incur retrieval fees if data is accessed frequently, making them less cost-effective when patterns are unknown. S3 One Zone-IA also lacks multi-AZ redundancy.