I am new here and i was like "retro OS"? This is my place! Then i went and read the posts and it was like OS=windows. So let me mention all the OS i have used so far, maybe you will find some interesting.
I started with DOS 5.0, then Win3.1, but after a year i have switched quickly to 95, then to 98, and to 98SE, which i used for a long period of time.
I have experimented with some linux then, but i never found it enjoyable.
Then in the last sec i have met with BeOS before Be Inc. went down, and used BeOS for a very long time. But i had to use some windows-only applications, so i multibooted.
For education i had to switch to WindowsXP on win front, and also upgraded to Zeta on BeOS front.
Then there was a short lived QNX 6.4 install, still with the classic Photon GUI. Also made an errand in the OS/2 world. Then there was a period where i have spent plenty years to hop from one OS to the next to find the ONE. But i haven't.
Then i started to hackintosh and used mac from 10.4 - 10.10 or so, but it went really downhill after SL.
After this i jumped to Haiku and running still today, but in the beginning it wasn't able to support me as main OS, so had to looks for a companion OS, so distrohopped almost every major and small linux distribution, but it felt like nothing changed in linux distributions in the past 20 years since i first met with linux (ok, configuring X wasn't a thing anymore, but can't really mention any positive change other than this). Then i had a long-living FreeBSD install, but then i have switched to OpenBSD and i think i can call myself happy now.
Of course there was plenty other small and strange OS installed on my computers, or used during day-job, like HP-UX, SGI IRIX, AIX, Plan9.
Show us your list!