NeXTStep/OpenStep
I've tinkered with lots of OSes including Plan9 and Amoeba. I'm very partial to NeXTstep/OpenStep. It came with Pixar Renderman and had amazing 3rd-party apps like SolidThinking (whose interface hasn't changed much to this day). All during the time of DOS/Windows 3.x. And developing in Obj-C/Interface Builder at the time was revolutionary.
I've tinkered with lots of OSes including Plan9 and Amoeba. I'm very partial to NeXTstep/OpenStep. It came with Pixar Renderman and had amazing 3rd-party apps like SolidThinking (whose interface hasn't changed much to this day). All during the time of DOS/Windows 3.x. And developing in Obj-C/Interface Builder at the time was revolutionary.