Honestly, if you want one simple DE for everyone it should probably be XFCE. Dead simple to use, feels vaguely familiar to Windows users, not overly complicated.
KDE is heavily customizable, Gnome is very opinionated, and tiling WMs don’t adhere to orthodox UI patterns. Those are all suboptimal if you want something usable by the absolute widest range of users.
Someone recommended Linux Mint’s Cinnamon also.
There’s also Pantheon from Elementary OS that I really really liked. But it’s missing a few features in my opinion.
Mind you, the real winner is of course Android. It has a consistent, easy to learn interface and a wide range of applications that integrate nicely.
And we don’t need to speculate; it has already won and is the true face of Linux for the masses. Plenty of young people don’t even own traditional computers anymore and do everything on their smartphone or tablets.
And that’s why this entire discussion is really just a form of fan wank; we don’t need to find a unified UI for Linux because it has already been found and has a massive market share. You may not like it but this is what peak performance looks like.
Everything else can be as complicated, janky, or exotic as it wants because it doesn’t matter.
We’re talking about desktop environments.
