OSDev.org

i personally use xmonad.

Edit: I switched to dwm because of the the low memory footprint

Used to use dwm, now on awesome. I had dwm on my last laptop, then I switched to a Tuxedo. which is already running some Linux distribution, and I have been too lazy to switch back. And while I thought dwm was cool for its size, I didn't want to deal with compiling it myself all the time, so I switched to awesome. I will never use a non-tiling WM ever again if I can help it.

Do WindowServer on macOS count? I do most of my coding under macOS.

(macOS is UNIX just like Linux, but it works with my 24-inch 4K monitor and hybrid SSD-HDD setup.)

I like good old Xfce.

I use TDE, I really dislike where Qt went with Qt4 and Qt5 so using TQt3 is an upgrade for me.

XFCE.

MATE, right off the default Linux Mint install.

GNOME 3. I've used tiling WMs in the past (I've used awesome for a long time) but nowadays I favor a desktop that just works without additional configuration.

Long ago I used to contribute to Compiz, and I continued using it long after the rest of the Linux world had moved on... these days I just live with Gnome 3.

Tried Fluxbox, Openbox, etc. and dwm, icewm and lxqt. I mainly use jwm now. I prefer working with several urxvt terminals open for multitasking and doing most things from the command line.

Lxde

In the "good old times" I used KDE 2.x and then KDE 3.x. After the KDE 4.x fiasco, I switched to GNOME and mostly stayed with it until now.
But in the last few years, I've been very disappointed with GNOME because of the decision to drop support for the desktop icons. Call me whatever you want, but I find the desktop icons very useful for temporary files or anything like notes. It's a big deal dropping them and there's no decent shell-extension to replace their original functionality. Ubuntu 20.04 was released with an embarrassing alpha-level extension for the icons. Only recently, the "Desktop Icons NG (DING)" was released, which is still not good enough IMHO, but at least does not cause crashes. In other words, I believe Gnome has taken a risky path and will likely loose many users for some of its decisions, as already happened in the transition GNOME 2 -> GNOME 3. GNOME 4.x wants to be fancy like MacOS's user interface, but not even Apple dared to remove the desktop icons. Trying to "innovate" at any cost is risky.

Now, on some machines I started to use again KDE, version 5.x and I realized that's great. Stable, fully-featured, customizable and it consumes very little memory, a little less than XFce!! As far I as know after doing some research, a few years ago KDE was a monster, but then the team decided to invest seriously in optimizing its memory footprint in order to make KDE more competitive and now it's pretty lightweight compared to GNOME. So, I'm now seriously considering to gradually switch back to it.

For a couple of years I think I had a crush on LXDE, but I abandoned it because it's not maintained well-enough and, apparently, it's destined to die, being replaced by LXQT, which is worse, at least for the time I've used it. Its essential for me a desktop environment to work smoothly and handle well all the corner cases.

i3wm with polybar and suckless terminal!

I use Open Shell for Windows 10.

What Ubuntu comes with. I personally like KDE the best for appearance, and XFCE overall.

Edit - I switched over to KDE, and love it! It is probably the best looking DE out there, besides maybe Aero from Windows Vista / 7.