Windows 7 or Windows 10. There's no other competition. You'll see slightly better performance with Windows 10 and you have directX 12 support that you won't have in Windows 7, but you might see better support for older games on Windows 7.
Basically, I suggest Windows 10 unless you have a specific reason to opt for Windows 7. Windows 7 has been the operating system of choice for gamers for years, but I'm seeing more and more of those gamers migrate to Windows 10 as newer games come out and directX 12 becomes more necessary.