Windows 8.1 was pretty solid, if short lived. New task manager, overhauled file copy, better SSD support & HiDPI scaling/multi-monitor support was hugely improved over 7, 8.1 also improved the window snapping and fixed the horrible table UI for it from 8.0

8.1 also ran faster than 7 on a lot of machines, they did a lot of work to optimize it for the crappy tablets which translated well to desktop.

Also the last version of Windows that you could opt-out of telemetry with (until Microsoft backported 10's telemetry engine to 7 and 8.1 via optional update)