I can tell you one thing that makes real changes to RHEL at least, CIS Benchmark. It hardens your system by tightening up file permissions, user logins, disables old protocols, sets partition flags and more.
But the best hardening imho doesn't follow any set standard, rather application dependent isolation using containers and MACs like SElinux and MCS (multi-category security).
https://docs.redhat.com/en/documentation/red_hat_enterprise_...