I think X actually came after the DOS-based Windows. If you're talking about machines I actually still power up and use, it's just MacOS and Linux (hence X). I run a virtual PC with Windows on my Mac for those occasional obscure programs that work only on Windows, and I have an X server running on my Mac, too.
Somewhere in a closet I have an Atari 800, does that count?
Of course Smalltalk on the Alto was the beginning of WIMP-style widget managers, and MacOS is its child. And the Smalltalk in which Scratch was initially implemented is another of its children.
But the first, very primitive window system I ever used was was WAITS (Western Artificial Intelligence Timesharing System) on the PDP-10 at the Stanford AI Lab. It only had non-overlapping, full-width windows that you could arrange vertically on your screen. The windows came in two flavors, a "piece of paper" that gave you scrollable text, with new text (from the keyboard or a program) always added at the bottom of the window and a "piece of glass" that didn't provide scrolling but did let you add text or bitmap graphics anywhere in the window.
That in turn was a child of Zeus, a Stanford timesharing system on the PDP-1 that provided display windows. They made a movie about it:
Sadly the quality of the video is terrible; this is a dub of a dub of a dub of a videotape. When I watched it at Stanford, you could actually read the code on the screen!
Now that everyone has display support in their OS, you'll wonder what the fuss was about if you actually watch the film, but back in 1967 most people were still using batch processing systems, and even the people with timesharing mostly had Teletypes as terminals.
P.S. Or you can read my paper about it: