How do you choose a Linux desktop for beginners? The answer is more complicated than usually admitted. Besides ease of use, you should also think about first impressions, the quality of help, stability, and room to grow. These characteristics immediately eliminate distros like Gentoo or Slackware, but still leaves dozens of alternatives, none of which rate high in all these characteristics.
Straight out of the starting blocks, I would eliminate distributions that try to make Linux look like exactly Windows. Anyone who is unhappy enough with Windows to look for alternatives is unlikely to want more of the same.
Nor is an enforced simplicity an answer. Both GNOME and Unity have tried to simplify the desktop, and neither has succeeded. In the wish to remove clutter, GNOME developers added the clutter of an extra basic screen, while Unity has become a desktop that might have been cutting edge in 1999 -- and proceeded to clutter key features with advertising and multiple filters. Although designed with newcomers in mind, neither has proved noticeably easier than other desktops.
Besides, several decades into the age of personal computers, you can probably take a degree of user experience for granted. Very few new users will also be new to computers.
What most users are likely to know is the classic desktop -- one with a panel, a menu, and icons on the work area. The Cinnamon, KDE, LXDE, MATE, and Xfce desktop environments are all classic desktops that should be easily learned by anyone. LXDE, the most slimmed down of these options, is almost certain to be quickly comfortable to anyone, and the rest are not far behind. However, having a classical desktop is only the beginning.
Although helping new users adjust is the main criterion, it is not the the only one. If the idea is to encourage users' transition to Linux, then a distribution that creates a strong first impression is important. Lubuntu, the LXDE version of Ubuntu, scores high in this category, and so does Mageia and Kubuntu. By contrast, as innovative as Cinnamon and MATE can be in features, their defaults suffer from anemic colors and general blandness. In fact, so do most distributions whose default wallpapers are an advertisement for themselves; first impressions are about the user, not the distribution or desktop.
Of course, many users eventually customize their desktop wallpapers and fonts, but the point is to entice them to stay long enough to reach that level of knowledge.
Another important characteristic for making a good impression is stability. Too many crashes and bugs will send potential converts scurrying back to Windows faster than anything I know, convincing them that Linux is still not ready for daily use. To avoid creating such impressions, I would currently avoid Kubuntu, because of ongoing problems with the Plasma desktop.
By contrast, an installation from the Debian Stable repository is generally so reliable that it is often used for servers. The tradeoff is that the packages are often not the newest available, unless a new release has just come out in the last few months. However, when new users gain experience, they can always borrow from the Testing or Unstable repositories if they prefer something current. Meanwhile, they should be having a disaster-free learning experience.
Another consideration is the accessibility of online help -- and whether the users you are helping will use it. Cinnamon and MATE score high in this category, beginning with a window full of links to forums and web pages that appears at every login until it is turned off. Some distributions also include a folder full of information, and many include Help buttons, but Cinnamon and MATE make help resources impossible to miss.
For adventurous users, I would also try to provide room to grow. A quick ramp up is important, but it is only a brief part of any user's experience. Although many users are content with a desktop whose sole purpose is to launch programs, others enjoy learning about customization options and desktop utilities once they have a basic understanding of the interface. Such users may eventually find LXDE limiting; by contrast, KDE should provide them with enough to explore for several years. Meanwhile, KDE's defaults are reasonable enough that they can ignore the advanced settings while learning the basics.
In addition, a distribution with advanced features shows Linux at its best. The most advanced desktops today all run on Linux, and, aside from free- licenses, are the most important features that a desktop or a distro can offer. Give users room to explore, and you will be presenting Linux in the best possible perspective -- to say nothing of introducing new users to what Linux is supposed to be about.
I am reluctant to name a single choice for everyone. Instead, I would start with what matters to the new user I was coaching, and narrow down the choices from there.
Yet, if I had to choose a single recommendation, it would be a recent Cinnamon or MATE release with KDE installed. This choice rates high in all the categories mentioned except first impressions, and I would probably improve that by immediately customizing it according to the taste of the user I was introducing to Linux.
However, I would prefer not to generalize more than necessary. I suspect that one of the many reasons for Linux's inability to establish itself on the desktop is the tendency to lump all new users together. Consider each user separately, and the chances of convincing them to stay with free software will probably become much higher.