When an object is eligible for multiple transitions, S3 chooses the transition to the storage class with the lowest storage cost.

When an object is eligible for multiple transitions, Amazon S3 indeed chooses the transition to the storage class with the lowest storage cost. However, determining which storage class has the lowest cost isn't always straightforward, as it depends on various factors including your specific use case, access patterns, and data retention requirements.

Generally, the S3 storage classes are designed with a cost hierarchy, where classes designed for less frequent access and longer retrieval times offer lower storage costs. The S3 Glacier storage classes, for instance, typically offer lower storage costs compared to the standard S3 classes.

Among the S3 Glacier storage classes, S3 Glacier Deep Archive is usually the lowest-cost option, followed by S3 Glacier Flexible Retrieval, and then S3 Glacier Instant Retrieval. However, it's important to note that these classes have different retrieval times and minimum storage durations that can affect their overall cost-effectiveness for your specific use case.

For example, S3 Glacier Deep Archive has the lowest storage cost but also the longest retrieval times (9 to 48 hours) and the longest minimum storage duration (180 days). S3 Glacier Flexible Retrieval offers faster retrieval options but at a higher cost, while S3 Glacier Instant Retrieval provides millisecond access at a premium.

To determine the most cost-effective storage class for your needs, you should consider not just the storage cost, but also factors like retrieval costs, minimum storage duration, and your data access patterns. Amazon provides pricing information on their website, which you can use to compare costs across different storage classes based on your specific usage patterns.

Additionally, you can use tools like Amazon S3 Storage Class Analysis to analyze your storage access patterns and help you decide when to transition data to lower-cost storage classes. This feature observes data access patterns and can help you improve your S3 Lifecycle policies to optimize costs.
Sources
Understanding S3 Glacier storage classes for long-term data storage - Amazon Simple Storage Service
Amazon S3 Features - Cloud Object Storage - AWS
How Amazon S3 handles conflicts in lifecycle configurations - Amazon Simple Storage Service