I thought MS-DOS version 3.3 hit the sweet spot on capability, performance, and memory usage.
As far as Windows, that honor should go to Windows 2000 (NT ver. 5.0) with no service packs installed. Of course, MS had to hire some adults from DEC to design NT for them.
I agree. I would also give an honorable mention to Win98se2 for 16 bit versions, for finally implementing USB, and cleaning up the horrific mess made by Win95.
I loved RSX-11M!
My recollection is that it could run on a 16K PDP-11 machine.
ML/NJ