I have been a Windows user for quite some time now (since 1995, to be exact). The only other major OS I used was iOS when I got an iPhone in 2008. But lately, I am using on daily basis a Samsung Galaxy S with Android 4.2 on it, as well as OSX Mountain Lion in my new Macbook Air.
Windows 7.
Still remains my favorite OS. I have adjusted my workflow over the years to be extremely efficient in this OS, by moving the taskbar to the left (for today's widescreen monitors) and adding various gadgets on the right.
Windows 8.
I installed Windows 8 in my laptop and desktop since last October (still keep Windows 7 at work). I appreciate what Microsoft is trying to do by unifying the tablet/mobile and desktop OSes, but it is not usable. I like the improved task manager and copy dialogs, but I never use the new start screen. Instead, I only use the desktop, where I have added extra software to bring back the start menu and the gadgets, i.e. make it look more like Windows 7. I also hate the flat graphics, without the shades and gradients (Aero scheme). It looks uglier compared to Windows 7.
OSX.
Beautiful OS, but I hate the font rendering they do. In no way does it look better than the Windows rendering in non-retina screens, especially in browser text. It appears blurry to my eyes, and there seems no way to fix it. I want to have OSX as my main OS, mainly because it is the only one that can correctly handle retina screens without having the fonts look ugly or of wrong size. I like the integration with the iOS in iPhone, and how the iCloud settings are transfered, including mail accounts. There are some software that I use that don't run in it, but I don't mind going back to Windows from time to time.
iOS.
This is by far the best OS I have used. It's friendly, rarely crashes, looks gorgeous on my iPhone 5, and you just wanna lick it. I think that iPhone 5 with iOS6 is one of the most astonishing engineering achievements of our era.
Android.
I read multiple posts lately about people switching to Android from iOS, saying that it is now a good enough mobile OS. That is definitely not my experience. It is not as consistent, the navigation is all over the place, and it looks much uglier compared to iOS. Aesthetics are a big deal for me, and I cannot use an OS that focuses only on functionality.