@DictionaryPlus OpenBSD because:
1) Simplicity : in the ideas behind it, the code, the tooling.. etc.
2) Consistency: release cycle, development practices, security practices, code, documentation.
3) Documentation: The manpages that ship with OpenBSD are second to none.
If your goal is to run OpenBSD, dive right in. Nothing can prep you for running OpenBSD as well as OpenBSD can :)