While technically not a Linux distro, Qubes OS is the gold standard. With the primary cons being that it's kinda hard on system requirements and it doesn't play nice with dedicated GPUs and thus software that would require it.
Honorable mentions would be Fedora Silverblue/Kinoite/Sericea, Kicksecure, openSUSE Aeon/Kalpa and Vanilla OS. Of course, regular Fedora and openSUSE Tumbleweed are still good even without being immutable. The aforementioned distros all have varying levels of hardening out of the box. While the offerings of Fedora and openSUSE have better defaults than most other distros, Kicksecure -which is made by the same team behind Whonix- is almost completely hardened from the get-go. Vanilla OS is in a major overhaul, so I refrain from making any strong judgements on it yet.
For whatever it's worth, a couple of years ago the (infamous) Madaidan (AKA security researcher on Kicksecure and Whonix) did recommend running minimalist distros like Alpine, Artix, Gentoo and Void for the sake of security. However, he did that recommendation on the basis of minimalism and zero-trust. However, that would require the system administrator (read: you) to actually know their shit. Which, unfortunately, is often times not the case as not everyone that's sensitive of their digital security proceeds to study cybersecurity. That's where the "honorable mentions" in the previous paragraph come into play; all of the distros that were mentioned within actually have shown to take security very seriously and acknowledge with the amount of heavy-lifting they do that they hold a sense of responsibility in that regard.
Im most interested in blendOS, because its based.
I once had an interaction with its primary developer and the dude was oblivious on which MAC was configured on his distro; spoiler-alert: none. It does a bunch of cool stuff, but I wouldn't call it secure (by default) by any stretch of the imagination.