I would say Window 7. I'm using Window 7 Home Premium for years and almost all games work on it. Well, I'm not able to play most games because of my graphic card - not my operating system. Window 8 might take over Window 7 title soon when more games support Window 8 system.
Windows 7 is probably the best. Windows 8 hasn't really been supportive of native games, let alone show up it's abilities like Windows 7 has. Most games are also supportive of Windows 7 and not Windows 8, even though that's a pretty rare occurrence.
Previously it was Windows XP, but now I think the most reliable is Windows 7 for numerous gaming applications and online gaming. They work best with the operating system.
I would say windows because that's where most games are being played on. Older games might have problems on windows 7 though. I am to sure about windows 10.
There will always be people with their own opinions on this. But I'm a diehard pc gamer. I used consoles up to the playstation 2 and then switched over. But even while I had consoles I was still playing computer games. Let me put it this way. I once played a game for so many hours straight that my monitor got so hot it caught fire lol.