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.
Among those qualifiers would also be 'free open source'. I'd think there are hardened operating systems which are not much, if at all, talked about in public. Perhaps, in the age of LLMs finding most security vulnerabilities, those are actually demonstrably more secure now than the well reviewed public ones.
... then there's Qubes OS, which beats the pants off AOSP isolation using Xen. Feels like the labelled desktop of the Trusted Solaris days! If one wanted to utilize SELinux in TemplateVMs for further hardening, that is always an option.
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.