Windows. I like OS X but it's too restrictive, and I enjoy playing video games on my . Plus my Macbook crapped itself barely a month after getting it, had to get the harddrive replaced and it's been slow ever since. I barely use it any more.
I'm a PC gamer so I'm stuck with Windows regardless. Ain't got time for running OS X when the majority of applications/games I use aren't natively supported for the operating System, though.