I am using Windows 7 Ultimate and it is one of the best.Windows 8 is preaty good but I don't like it to much for games.It is more for business than for gaming.
If u want to play games than windows 7 (proffesional or ultimate).If u have lap top and use it for internet only i think win 8
win 8.1,but i think new win 10 will be better optimized for heksa core processors or better,and people who have that kind of processors will have much more fps in games.
I prefer windows 8.1 cause he is much faster(if u have normal computer).If u have bad computerthat take windows xp or if wanna play some games what not support on xp take windows 7.
XP all the way man, old school (it takes small amount of ram memory) while newer versions eat ram like a cake
Windows xp is good for gaming because it takes 100 mb of ram but with good ram u get better perfomance on windows 7 i dont like 8...