- VPC - basic building block for any AWS-based infra that isn't ancient
- CloudTrail - only way to get audit logs out of AWS, no matter what you feed them into
- CloudWatch - similar with CloudTrail, many things (but not all) will log to CloudWatch, and if you use your own log infra you'll have to pull from it. Also necessary for metrics.
- ELB/ELBv2/NLB/ALB - for many reasons they are often the only ways to pull traffic to your services deployed on AWS. Yes, you can sometimes do it another way around, but you have high chances of feeling the pain.
My personal typical set for AWS is EC2, RDS, all the VPC/ELB/NLB/ALB stack, Route53, CloudTrail + CloudWatch. S3 and RDS as needed, as both are easily moved elsewhere.