{Safest Operating System}

It depends. Every OS has its own up's and downs. If you are really going behind something that locks you down you might want to look into a few BSD flavors like OpenBSD, FreeBSD, etc. OpenBSD does shine itself in the security segment.

OpenBSD period.

GNU maintains a list of libre OSes. I can't access the site right now, but I believe this is the proper link. Edit: It is.

Free distros

"Safest" is a relative measure. However, the ones listed at the link above only come with "free" software. "Free" meaning you have the freedom to see the code, modify it, and share those changes. Which means that collectively, users can make sure that the software isn't sacrificing privacy. Unfortunately, a lot of popular software that you might want to use is not free in these ways, so it may not be available by default. And you might have to hunt for alternatives.

The Mars rover (among others) uses VxWork. Highly stable. Only does what you programme it too, so you're in complete control of user's privacy.

Maybe

• qubes-os.org/
• grapheneos.org/
• tails.boum.org/index.en.html

Debian Stable.
debian.org/