I've only got my hands on Windows, Mac OS and Ubuntu Linux. Haven't really played with Ubuntu yet (it's dual booted on my laptop and I rarely use that)
I really enjoy the look and feel of OS X, especially now with Yosemite (inb4 someone calls me iSheep ;c) but I've used Windows for most of my life. Overall I prefer Windows because I can play many games on it as well as many creative programs and I am just more used to Windows features and such.
I miss WinXP and 98, but Win7 will do.
Note: Can I get opinions on Win10 from anyone?