Debian - The most widely used base of all Linux distributions.
"The Debian Project is an association of individuals who have made common cause to create a free operating system. This operating system is called Debian. Debian systems currently use the Linux kernel. Linux is a completely free piece of software started by Linus Torvalds and supported by thousands of programmers worldwide. Of course, the thing that people want is application software: programs to help them get what they want to do done, from editing documents to running a business to playing games to writing more software. Debian comes with over 50,000 packages (precompiled software that is bundled up in a nice format for easy installation on your machine) - all of it free. It's a bit like a tower. At the base is the kernel. On top of that are all the basic tools. Next is all the software that you run on the computer. At the top of the tower is Debian -- carefully organizing and fitting everything so it all works together."
The above text is a direct quote of the distrowatch description and to my mind at least is quite an apt description. A few words of warning though for users who are not already familiar with this Operating System...
This OS is definitely NOT a beginners system, although it can be easily installed it has some limitations that many (or most) of the "derivatives" do not have, one of the most important of the "missing features" is the lack of firmware for wifi devices, which is probably the most common of new user complaints. Because of the philosophy of Debian it does not contain any "non-free" software, this includes firmware which is often required by wifi devices before they will work. This can be a huge obstacle to anyone unfamiliar with Linux and the way it works but once a few fundamentals have been learned it can be relatively easy to get the systems up and running.
With that said, I have found that Debian has been the most robust and error free of all the Linux offerings out there, security updates are implemented regularly and the "Stable" version (Stretch at time of writing) is one of the most stable I have used. If you are intending to learn the Linux Operating System then Debian would be my recommendation, if on the other hand you just want a free desktop operating system that requires very little (if any) system maintenance and knowledge then I would advise you to choose one of the many derivative options available, Q4OS, MX Linux and Sparky Linux are a few of the good alternatives.
Whichever version you decide to install, be sure to visit their websites and read some of the documentation that is usually available and if you have any issues you can usually get the best help in the user forums which are linked from the respective websites.