OpenBSD is the most security hardened OS out there, and the project maintains many pieces of software people take for granted, like pf, openssh, and libressl. FreeBSD is definitely the most robust, and is a very good server OS. NetBSDs niche is how many platforms can run it. If it's supported by GCC, theres a decent chance NetBSD will run on it
OpenBSD is the most security hardened OS out there,
I think this requires a lot of additional qualifications. For instance, AOSP and Android are much more hardened when it comes to isolation between GUI applications.
Unfortunately AOSP is owned by Satan, which does not seem to be the case with OpenBSD.