Adding to this with some applicable guidance whether it's Linux or BSD. The most secure OS would be one where you have an understanding of all it's security co

Adding to this with some applicable guidance whether it's Linux or BSD. The most secure OS would be one where you have an understanding of all it's security configuration options, have those things configured, and verify that configuration on a regular basis. With Unix-like operating systems you have the advantage of many of these things being controlled via text-based files, so you can version-control these and detect deviations of the configuration. It also worth mentioning that the most secure OS would be supported by a network architecture that isolates it reasonably well. Firewalled subnets using the concept of Zero Trust are valuable. On the application side, ensure whatever is running on these servers is minimal. Complexity can mean vulnerability, whether it be from the application itself or it's build dependencies. Minimalism is another strength Unix-like operating systems can deliver.