Linux, Windows + other OSs

Tails - The Amnesic Incognito Live System. Tails is a live system that aims to preserve your privacy and anonymity. It helps you to use the Internet anonymously and circumvent censorship almost anywhere you go and on any computer but leaving no trace unless you ask it to explicitly.
Whonix - A High Security Method of Surfing the Internet. Whonix is a desktop operating system designed for advanced security and privacy.
QubesOS - Qubes is a security-oriented, free and open-source operating system for personal computers that allows you to securely compartmentalize your digital life.

Kali Linux - /r/KaliLinux - a Debian-derived Linux distribution designed for digital forensics and penetration testing.
Parrot OS - /r/ParrotOS - a Linux distribution based on Debian with a focus on computer security. It is designed for penetration testing, vulnerability assessment and mitigation, computer forensics and anonymous web browsing.
BlackArch - an Arch Linux-based penetration testing distribution for penetration testers and security researchers.

Debian - The Universal Operating System
FreeBSD - FreeBSD is an operating system used to power modern servers, desktops, and embedded platforms.
Ubuntu - Ubuntu is an open source software operating system that runs from the desktop, to the cloud, to all your internet connected things.
Fedora - Fedora creates an innovative, free, and open source platform for hardware, clouds, and containers that enables software developers and community members to build tailored solutions for their users.
CentOS - a Linux distribution that provides a free, enterprise-class, community-supported computing platform functionally compatible with its upstream source, Red Hat Enterprise Linux (RHEL).
Windows Server 2019

LineageOS - /r/lineageos - A free and open-source operating system for various devices, based on the Android mobile platform.
GrapheneOS - /r/GrapheneOS - GrapheneOS is a privacy and security focused mobile OS with Android app compatibility.