What is the MOST operationally efficient way to determine the Region in which the template is being deployed?
Use the AWS::Region pseudo parameter.
Require the Region as a CloudFormation parameter.
Find the Region from the AWS:StackId pseudo parameter by using the Fn::Split intrinsic function.
Dynamically import the Region by referencing the relevant parameter in AWS Systems Manager Parameter Store.
Explanations:
The AWS::Region pseudo parameter automatically returns the region where the stack is being created. This is the most efficient and simplest way to obtain the region.
Requiring the region as a CloudFormation parameter adds unnecessary complexity and manual intervention for each deployment.
The AWS::StackId pseudo parameter provides the stack ID, but using Fn::Split to extract the region would involve unnecessary complexity and is not the most efficient method.
Referencing Systems Manager Parameter Store adds operational overhead and is not required to obtain the region, as AWS::Region already provides it.
I rate that the answer is:
Use the AWS::Region pseudo parameter.