I’m not a developer neither an advanced Linux user and these are just my opinions at the moment, they might change in the future and might not make sense to everyone, this is from a user perspective:
Easy to use, some nice features, it’s a bit heavy if your computer is old. Never used for enough time to see if there are many bugs or not, so I won’t comment.
Very simple and easy to use, anyone that has used a smartphone before will know how to use it. You can even use with touchscreen, something that will be more common in the next years because of convertible laptops.
Removing features(like task bar and desktop icons) and replacing them with extensions have the advantage of less things breaking(bugs) for the average user and focusing more in the rest of the DE. Most people will not bother installing extensions because they usually stick to defaults. Sadly a lot of extensions are not updated and some don’t work.
Animations are still a bit slow and stutter for older computers, they will probably fix this in the next years. It’s a bit heavy(~1GB RAM) but in 5-10 years almost every computer will be able to run it with no problems because old hardware with 2 GB RAM or less and simple CPUs will break at some point.
Tons of features and customizations possible, not recommended for beginners, it’s not for everyone(some people like, some people don’t). It was heavy but now it uses something like 400 MB RAM idle, it has improved a lot.
The main issues: bugs and GTK apps(KDE uses Qt). I was using 5.12.x recently and KDE has improved a lot, they are doing a great job. As more people use it, the bugs will become less frequent and less serious, more eyes = less bugs.
Very light but kinda ugly and not enough features. The whole discussion about RAM/CPU will be pointless in 10 years from now because almost all computers will have at least 4 GB, even a Raspberry Pi has 1 GB RAM now and almost 1,5 Ghz. I don’t see the point of using them right now, only if you have a very old computer with like 10 years old or want extreme performance on a new computer. In the future very lightweight systems will not make much sense.
Promising, but too new yet, I feel it’s not ready yet, but they are improving fast, it’s light and has interesting customization options. MATE Tweak makes it easy to emulate other Desktop Environments.
Too new(something like 5 years or less), I think they still need a few years of development and honestly I don’t see the point of using them right now.
It’s light and it works, but doesn’t has lots of features and has a slow development. I hope I’m wrong(because I like Xfce) but I think the project will not be maintained in 10 years from now, I think maybe MATE will replace Xfce at some point as it gets better.
So, the 3 best DEs in my opinion right now, for me and probably for most average users and beginners(alphabetical order):
Important: if you want to test other Desktop Environments, the easiest way is to use a virtual machine with softwares like GNOME Boxes, VirtualBox and similar ones. Another good advice is to search and see some images(screenshots), videos, blogs and podcasts before installing them.