Which OS better for gaming?

Well Windows 10 is great if you have a computer that can handle it easily. Don't jump on me, "tHiS iSn'T 2005 aNyMoRe"-squad, there are people still playing on old machines, playing WoT, LoL and CS:GO mainly.
Windows 10 does a lot of crap in the background and uses lots of RAM, downloads updates on its own and sends diagnostics data now and then. Checks for updates and bakes cookies. Have I mentioned its full of bloatware?
All this can affect your gaming experience.
Windows 7 still holds up great, and you won't miss on anything. Its stable, finished, and really well done. If you have a low to mid end computer you might gain a bit by using Windows 7, but it varies from user to user.
Windows 10 is newer and has great features but Windows 7 is better resource wise.

My opinion.