This would best be asnwered by a persons needs, for example if you want to game and not very good with computers with a budget then Windows.
If you want to game want a OS that is stable with ton's of features and easy to use with a reputation of getting better with each release then OSX, whether you buy a apple computer or you build and install OSX.
If you do not game, want ton's of features, easy to use and continually get's better/more support with each release then I would say Ubuntu (or any Linux distro for that matter).
I do very lite gaming (games by blizzard or valve which run with no problems) and for me it is Ubuntu, it runs far smoother then windows 7 on my pc and is able to multi task far beyond what it did when running windows. There are so much free software out there I can't find a reason to go back to windows. From video/picture editing/organizing to music production to organizing/syncing my music to my mp3 player. Plus you can completely change the way your OS interface looks to better fit your use. And everything is easy to use. Yeah I have had to stop using certain software that I used on windows but I have found a Linux replacement for everyone of them.
This is just a opinion.