You are architecting a cloud solution for a financial firm where the application should be able to handle thousands of financial transactions per second with single-digit millisecond latency. Which GCP database service would be most suitable for this requirement?

*Explanation:* To meet the high throughput and single-digit millisecond latency requirement for handling thousands of financial transactions per second, the most suitable GCP database service is **Cloud Bigtable**. Cloud SQL may not provide the necessary throughput and latency for high-velocity financial transactions. Cloud Spanner, while globally distributed, may not meet the single-digit millisecond latency requirement. Cloud Datastore is not designed for such high-velocity, low-latency demands, and Firestore may not meet the same latency and throughput requirements.