Qubes OS - A reasonably secure operating system.

Edward Snowden on Qubes OS - https://twitter.com/Snowden/status/781493632293605376

What is Qubes OS?
"Qubes OS is a security-oriented operating system (OS). The OS is the software that runs all the other programs on a computer. Some examples of popular OSes are Microsoft Windows, Mac OS X, Android, and iOS. Qubes is free and open-source software (FOSS). This means that everyone is free to use, copy, and change the software in any way. It also means that the source code is openly available so others can contribute to and audit it".

How does Qubes OS provide security?
"Qubes takes an approach called security by compartmentalization, which allows you to compartmentalize the various parts of your digital life into securely isolated compartments called qubes".

How does Qubes OS compare to running VMs in a conventional OS?
"Not all virtual machine software is equal when it comes to security. You may have used or heard of VMs in relation to software like VirtualBox or VMware Workstation. These are known as “Type 2” or “hosted” hypervisors. (The hypervisor is the software, firmware, or hardware that creates and runs virtual machines.) These programs are popular because they’re designed primarily to be easy to use and run under popular OSes like Windows (which is called the host OS, since it “hosts” the VMs). However, the fact that Type 2 hypervisors run under the host OS means that they’re really only as secure as the host OS itself. If the host OS is ever compromised, then any VMs it hosts are also effectively compromised.

By contrast, Qubes uses a “Type 1” or “bare metal” hypervisor called Xen. Instead of running inside an OS, Type 1 hypervisors run directly on the “bare metal” of the hardware. This means that an attacker must be capable of subverting the hypervisor itself in order to compromise the entire system, which is vastly more difficult.

Qubes makes it so that multiple VMs running under a Type 1 hypervisor can be securely used as an integrated OS. For example, it puts all of your application windows on the same desktop with special colored borders indicating the trust levels of their respective VMs. It also allows for things like secure copy/paste operations between VMs, securely copying and transferring files between VMs, and secure networking between VMs and the Internet".

How does Qubes OS compare to using a separate physical machine?
Using a separate physical computer for sensitive activities can certainly be more secure than using one computer with a conventional OS for everything, but there are still risks to consider. Briefly, here are some of the main pros and cons of this approach relative to Qubes:

Pros
Physical separation doesn’t rely on a hypervisor. (It’s very unlikely that an attacker will break out of Qubes’ hypervisor, but if one were to manage to do so, one could potentially gain control over the entire system.)
Physical separation can be a natural complement to physical security. (For example, you might find it natural to lock your secure laptop in a safe when you take your unsecure laptop out with you.)
Cons
Physical separation can be cumbersome and expensive, since we may have to obtain and set up a separate physical machine for each security level we need.
There’s generally no secure way to transfer data between physically separate computers running conventional OSes. (Qubes has a secure inter-VM file transfer system to handle this.)
Physically separate computers running conventional OSes are still independently vulnerable to most conventional attacks due to their monolithic nature.
Malware which can bridge air gaps has existed for several years now and is becoming increasingly common".

What about Subgraph OS? Well, it certainly shows potential but it is very much still in Alpha-testing stage... https://micahflee.com/2017/04/breaking-the-security-model-of-subgraph-os/

A good overview video of working in and with Qubes OS.

Qubes OS Hmoepage - https://www.qubes-os.org