Windows 10 is probably the new best, with directx 12 and more ram support, you should see some better performance in the future.
But windows 7 and 8.1 are still great, considering user experience and all that, but if you want the best performance now and in the future, you should probably get windows 10, especially if you don't care about those privacy issues.
Personally I just went back and installed windows 7 from 10 last night. 10 was nice,. but a lot of issues gaming, may be Windows 10, or Nvidia's windows 10 driver. Either way games seems to crash alot more than Windows 7. Now that I'm back on 7 been playing my games for hours with no issues.
Plus if I understand right you can buy Windows 7 or 8.1 now, and still upgrade to 10 for free till July 2016.
Personally I think it's for the best. Windows 10 is awesome, really fast, booting up and shutting down is quick, but if gaming is your main thing like mine, its a hot mess right now. Nvidia put out a hotfix and a new driver withiin 3 days, and I still couldn't finish a multiplayer match of BF 4 without the display driver crashing. Also games seem to have issues with Alt/Tab, they would just go to black screens with sound in the background.
if you upgraded from win 7 and didnt do a clean install there are much less in the way of issues.
personally ive had none more sevier than nvidias gforce experience telling me my system is below spec for optimization. no driver issues, bsods or any of that.
i did notice however in the last 2 months or so of using win 7 things started to slow down.
it turns out that m.s were giving out bad updates that were corrupting parts of the o.s.
shadow copy for instance would constantly try to make restore points for every file when you tried installing software. i went from a 5 min install for skyrim to nearly an hour till i found the problem and disabled the shadow copy service.
then there was just basic file management. my hdds seemed to be taking longer and longer to respond. no matter how often i defragged them access was just not as snappy as it was. ad thats on 2 raid 0 drives pushing 250+MBps. so yeah i think m.s were playing dirty in the weeks leading up to win 10s launch.