Linux

Gnome by default isn't customizable, you have to install and turn extensions on, but its as simple as toggling on chrome addons, you goto the extension store and grab what you want, the store could be better usability wise, hate the ui, flathub is like the only usable ui for a store on linux, insane how its all stuck 20 years backwards. (tho I kinda like it lol)

I think gnome just feels good and is the most usable, but thats because I feel the constant need to tinker with plasma, while with gnome I just set my extensions on, tweak them a little maybe once if they have settings and don't think about it again. It only takes a few to get gnome feeling personalized and it just works well. Plasma feels better swapping from windows, but gnome feels more like a different modern os on par with windows/mac but more minimalist.

I wish plasma was slightly less buggy, anytime I mess with a panel I have to clone a backup just in case it tweaks out and I need to delete it.

No desktop environment. Get a compositor, a runner/menu/app-grid, maybe a panel or dock, set some shortcuts, done desktop environment. It's how most of wayland outside KDE/Gnome currently works.

If infinite customization is what you're after you shouldn't use a DE. A WM like i3 och hyprland is much better suited for that

KDE has the most options out of the box. You can make it look like Gnome, or act like a tiling window manager, or like Windows 7, 10 or 11, just with the options it contains from the start.
Gnome comes with almost no options. If you add extensions, or know enough to make your own, the sky is the limit. But I wouldn't call that "customizable", you can write your own themes for Plasma, too.
Xfce is another one that's very flexible. But it's very hard to get it to look and feel modern, it will always be an old school desktop, no matter what theming and added docks you throw at it.

Probably Emacs. /j

KDE if you want to just configure stuff. Gnome if you want to code or manually style stuff.

I customised Xfce a lot, only with menu settings. I removed the window tabs from the status bar, the focused window title is written on the status bar. The window manager was removed for bspwm. The result is an optimized screen space while keeping the convenience of a DE.

KDE fs

Or roll your own via a compositor and various tools a la Hyprland

They're all exactly as customizable as you are willing to alter them.

I would say XFCE and Cinnamon; no two XFCE's look alike and Cinnamon can easily be molded into something very different as well.

I see a lot of people recommending KDE and Gnome; I've found those surprisingly rigid, although there are more guides on how to “rice” KDE into the most non-KDE things so there's that.