From what I can tell NixOS still has no support for AppArmor or SELinux.
Fedora at least tries relatively consistently—and has been for decades.
All those would be important, but don't let the perfect be the enemy of the good. We could start from the Vulnerability Of The Year: supply chain attacks.
Desktop Linux is very bad at that compared to ChromeOS and MacOS, but some distros are much better than others. For a few decades now, Fedora has been one of the better ones.