Windows 7 all the way. Windows 10 still has issues, especially if you want to have more control over your system. Windows 10 automatically downloads and installs updates, this is good unless your trying to run your system as stable as possible where you want to wait and check the updates manually.
Windows 7 takes up more disk space as it doesn't contain compression, Windows 8.1 does contain compression but Windows 7 doesn't. Windows 7 is also less secure than Windows 10.
Windows 10 is immune to a lot of the exploits used against Windows 7 systems, and contains better security overall. This combined with the automatic updates does make Windows 10 a good choice if security is your top priority. It's just less compatible with certain security software that might try to perform certain actions.
I recommend Windows 7 personally, but as I stated it's personal preference. Windows 10 is just a bit of a mess right now, and while it isn't a failure it's best to stick with Windows 7 for the time being.