Enabling multi-factor authentication (MFA) on all user accounts is a security best practice because it requires both the user's credentials and access to a physical device that generates a time-based code, significantly reducing the risk of unauthorized access. Rotating IAM credentials refers to changing access keys and passwords, which is also a best practice but does not involve the use of a physical device. IP whitelisting and enforced password complexity can enhance security but do not provide the same level of protection against compromised credentials as MFA does.