Ubuntu or mint both are fine

security or ease , pick one

OpenBSD

ducks

Qubes OS gives him high security with relative ease.

Fedora Silverblue with auto update and Flatseal tightened apps is a nice middle ground.

RHEL minimises supply chain attack risk and provides features like kernel hot patching. He can use free developer subscriptions. Also try SUSE.

Security wise Chromium is a bit better than Firefox. Try to seal it up with SELinux. Red Hat only supports Firefox however.

SecureBlue can be used as a reference, but it’s still downstream so personally I’d avoid using it in case of supply chain attacks unless securing Silverblue is too much of a hassle.

Keep in mind that Flatpak sandbox interferes with browser sandboxes.

If you’re picking a distro for someone else I would not recommend a small project distro or something incredibly niche 😅

Any of the big projects should be decent. Fedora, maybe fedora silverblue or whatever their imutable variant is called, opensuse, Mint, Ubuntu, debian. (Personally I don’t like some of the choices Ubuntu makes but it may still be a very good option for less technical folks)

Others can tell you which of those have the best security defaults, but to be honest it doesn’t sound like you actually have particularly exceptional security needs relative to what any distro will provide. I’d prioritize something stable and user friendly- which, again, your best bet is NOT picking a niche small project or something most people have never heard of

Based upon your wording, I am assuming your father is not particularly tech savvy, if this is the case first and foremost you should be picking a distro that is maintained by a large group of trustworthy developers, this removes the niche distros from the running. Secondly, since he isn’t going to want to learn the terminal, you should be picking a distro that installs programs with a GUI package manager or flatpak manager, this removes the likes of arch, gentoo, & open suse tumbleweed. Thirdly, you will want a distro that is based on one you understand well enough to run tech support, I don’t know which that is for you, if it is Debian based stick with mint, fedora based go with fedora workstation or fedora KDE, if it is opensuse I don’t have any recommendations sorry.

After you select the distro you need to educate your dad that he should only be getting new programs through the package manager, and I would either tell him the inherit insecurity of some flatpaks or remove flathub from your mirror list unless there is something he really needs in which case you need to do your research.

In general security on Linux is a lot more active for IT than it is for Windows, but for the general user if they can get by using a well known distro’s repos you shouldn’t have any security issues.

If you are overly worried you could add apparmor to the system to isolate the system from programs or pick an immutable distro like bazzite, but in general the immutables are smaller teams which is why I don’t prefer them.

Secureblue is what I’d use if security was a major concern. Every time I’ve tried to use a non-Ubuntu distro I’ve immediately ran into a few technical issues so I stick with Ubuntu.

Generally I think I’m safe as long as I don’t install untrusted software, and the distro didn’t package untrusted software.

OpenSUSE is big on the security and usability front. None of the services you install activate by themselves. Firewall active by default. The first user doesn’t get access to every group under the sun after installation.

And everything can be controlled through GUI tools. But it doesn’t throw a fit when you’ve done something yourself through the CLI.

OpenBSD. No Linux, but much more secure. And yes, there is quite some amount of Linux-specific malware around these days.

I think most Linux distros will be fine. As of today desktop marketshare is still small, the governments mostly work within custom business applications. And to this date Linux malware and viruses for the desktop are practically unheard of. The common attacks are against the browsers, not the underlying operating system (so do timely updates and install an adblocker) or we’d expect phishing or phone scams and that’s against the human in front of the computer, again not the operating system. That makes me say they’re about all alright. Of course they’re not all equal. Immutable distros and sandboxing will help here. But the real deal is other countermeasures, like be aware how phishing works and try not to mix online banking and pirating games from shady websites. That belongs on separate user accounts or even installed operating systems. And use password managers, 2 factor authentication and these things. (And don’t use Edge, or some browser from some random third-party repository.)