Given the requirements of ACID compliance and 99.999% availability for a global transactional application, Cloud Spanner is the most suitable choice among the options provided. Cloud Spanner is a fully managed relational database that offers horizontal scalability, strong consistency, and high availability across multiple regions, making it ideal for global applications requiring these features. Firestore is a NoSQL document database that does not fully support ACID transactions across documents. Cloud SQL is a managed relational database service that supports ACID transactions but does not offer the same level of global scalability and availability as Cloud Spanner. Bigtable is a NoSQL wide-column database optimized for large analytical and operational workloads but does not support ACID transactions.