I've lived through every evolution of Windows from 3.1 up to 11 and Millenium/2000 still remains my favourite and I will always consider it the most 'get-shit-done' UI that Microsoft has ever built. Up until W10 removed the feature, I used to turn off the Themes service so that I could get the classic UI back.
And I also completely agree with your point that everything else since then has felt like a poorly placed theme on top of something else.
Wait a sec. Windows 2000 was probably their best operating system. Windows ME was absolutely their worst. They were so different I’m not sure the entire company wasn’t swapped into the set of Severance (tv show) or something of that ilk.