If playing games, Windows 10. All hardware drivers will all work on it these days which is nice.
If you just need Windows to run some Windows software that isn't gaming, Windows 7 should be fine. The later updates will annoy the fuck out of you to upgrade to Windows 10 though.
These tools can be useful if you use Windows 10:
https://bgr.com/2015/08/14/windows-10-spying-prevention-privacy-tools/I'd suggest blocking all that Microsoft Crap with the hosts file. I also suggest blocking those TCP ports used for Telemetry because it's just a waste of bandwidth and pretty awful with Privacy.
You can do this to kill Cortana too if you want; it's fairly useless:
https://superuser.com/questions/949569/can-i-completely-disable-cortana-on-windows-10I did a few other stuff with my Windows 10 machine too but I can't remember everything. Was quite a lot of shit.
No need for 8.1 as there is no real reason to use it anymore.