3 OSs that I remember fondly:

* MS DOS 6.x: I played so many DOS games back in the day, I did how coding projects in Turbo PASCAL and Quick BASIC too and I have lots of fun memories of DOS back then.

* RISC OS: Acorn computers at school used RISC OS and I loved it! It looked so futuristic compared to Windows. Fun memories again.

* Windows 3.11 for Workgroups: I used this version of Windows a lot and even worked during one summer setting up computers at a lab with Windows 3.11 for Workgroups.