Debian. Truly the universal operating system. Runs on all of my laptops, desktops, servers, and NAS with no fuss and no need to keep track of distro-specific differences. If something has a Linux version, it probably works on Debian.

Granted, I am a bit biased. All of my hardware is at least 5 years old. Also came from Windows, where I kept only the OS and browser up to date, couldn't be bothered with shiny new features. A package manager is already a huge luxury.