Final answer:
Amazon CloudWatch is best suited for collecting important metrics from AWS RDS and EC2 instances, as it provides detailed monitoring and management for AWS cloud resources.
Step-by-step explanation:
To collect important metrics from AWS RDS and EC2 instances, the most suitable AWS service is Amazon CloudWatch. Amazon CloudWatch provides monitoring and management for AWS cloud resources and the applications you run on AWS. You can use CloudWatch to collect and track metrics, collect and monitor log files, and set alarms. Specifically for RDS, it can monitor database instances, and for EC2, it can monitor the instances' CPU utilization, disk I/O, and network utilization, among other metrics.