I would say it depands on what you use your computer for and how old and what kind of system you got. There are different kind of users who will prefer different operating systems. So it's a bit hard to clearly state "this" is the best OS.
Mac users are in general stuck with Mac OSX, so they will probably say it's the best. Lot of "expert" users, developers and administrators are likely to go for some kind of Linux distrubition. But for the average user who just likes to play games and watch pron online Windows is for sure the best choice. You only left with the question, which version of Windows.
On my old Pentium III en 4 i used Win XP (and different versions of Linux for testing purposes), and it always worked fine, did everything i wanted it to do.
On my new system i have been using Windows 7 64bit version for a while and i have to say it runs way smoother than XP, although i still like XP's interface better (but thats just a matter of taste). I would say for the average user, with a pc not older than two years, who plays games a lot Windows 7 is surely the best choice. For older computers XP is still the best option.
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