Well, until recently, I was a huge fan of Windows XP. The amount of flexibility and usefulness the OS had was really good, and when I needed to do some really productive stuff, I also used to dual-boot Ubuntu.
Now, in 2012 (well, it started in 2011), I upgraded to Windows 7, a bit afraid as to what I would experience with the new OS (especially considering I have a Pentium IV in my rig), but it ran like magic. Windows 7 runs just as fast as XP, if not even better, with nice support for DirectX 10 and 11. I know I won't be going to another OS anytime soon thanks to the power and flexibility of Windows 7 Professional.
(Side note, I do have to get a Macbook for college because I'm going for music education, so eventually, I'll be running whatever is the current OS of most Macbooks.)