Windows 2000 pro was peak win OS for me. I’d actually still run that now if it was possible.
The thing is while I don’t like the new UI I totally get that this is subjective choice. In that there will be many that do.
I’ve thought of another area which I’d like to see improve before I’d consider rejoining which is CS. It’s fine when things don’t go wrong but (in my experience) beyond terrible when it does. Inconsistent, helpful but always involving a specialist and no real time support on the occasions that you really do need them.