Windows 10
Checking the site for gaming compatibility on Windows 7 or 8
10
You can purchase the more affordable version and then upgrade at no cost for DirectX12 compatibility with Windows 10.
Won 7, but likely aim for 8; it doesn't really matter. Upgrade to Windows 10 once DX12 arrives.
I was saving to buy Win 8 code from g2a for around 30€.
10.
I’ll mention Linux because I really like it, but if you prefer Windows, go with Windows 8 for gaming.
It seems version 8.1 is significantly lighter than 7, which in theory would make it more suitable for games.
Does your PCMR have a profile picture?
Windows 7 discounts are available on G2A. You can also upgrade to Windows 10 at no cost starting from version 7.