Favourite OS has to be Windows 7 x64. I run OS X ML in VM for Xcode etc and I have given Ubuntu tons of chances over the last 7 years; I just always come back to Windows.

Windows has all the applications I want - my CAD software and suspension geometry software for Uni; I prefer the layout and look of the OS over Mac and Ubuntu; every single piece of hardware I ever buy already has fully functional drivers built; there's tons of reasons why I prefer Windows over everything else.

Another big reason is that having to use the command line for simple tasks in Ubuntu is an absolute pain in the arse. I'm perfectly comfortable with using any CLI, I just feel I shouldn't have to be inconvenienced by having to use one to do something like install a new package or an update.

OS's I use in order of preference:

#1. Linux with KDE: For work, this can't be beat. KDE has insanely good virtual desktop support. You can multitask heavily and still be very efficient. I grew up with CLI's, so I view a good one as a pleasure to use, not a hindrance as some do. Customizability is nearly infinite. It's generally less work for me to get a Linux box set up to acceptable productive levels than a Windows box.

#2. Windows 7: This runs my HTPC and I use it for games, movies, etc.. There's pretty much no decent Bluray support on any non-MS OS, and I do like bluray! Win7 still has some bugs, but it's a very livable OS.

#3. Windows XP: Yup. Ancient! However, a lot of our lab gear has drivers that won't work in anything else. I use it because I have to.

OS's I have used and am glad not to anymore:

OSX. I had OSX on my laptop for most of last year. It's nice in that it has better interoperability with Linux software than Windows thanks to it's BSD roots, but if you want to use Linux you might as well use Linux!. It technically has virtual desktops, but management is primitive. I've never liked the OSX dock (Give me KBar or even the Win95 start-bar any day!). Bluray support (one third-party app that does key management over the internet!) is dodgy and gaming is still not so good, so it's not really a viable HTPC OS. Lion was also very buggy. Much worse than Windows 7. Customizability is the worst of any modern OS. Apple tries very hard to force you to do things their way. Oh, you can change things, but it's usually more trouble than its worth. I expect the OS's I use to be capable of adapting to me rather than forcing me to adapt to them. OSX is ahead of the game when it comes to trackpad gesture support, but hopefully other OS's will catch up fast.

Linux. I was a Windows guy but from last two years I'm using Ubuntu. I don't have any driver issues and everything works quite well. In fact, it works better than Windows 7 for me. I'm primarily a developer and it is well known that Linux is best for that purpose. I am also a part time designer and GIMP/InkScape works well for most of my needs.

I still have Windows 7 installed, but I only use it(occasionally) if I need to work on things like .net, photoshop, illustrator etc.

Ubuntu. I've found that a decent amount of software for development is just harder to install on Windows. The open source community seems to mostly be on Linux, and they often have the best support and installation processes for Linux.

For example, setting up Emacs and getting the packages I want installed has been much easier on Ubuntu than Windows. This has also been true for a lot of other great open source software.

Basically, I just need my OS to be a unix. On the desktop I also want to have great UI/UX (font rendering is part of it. Can't stand for weird font rendering on Windows, things are better on Ubuntu though.) So OS X is my choice.

I also like such aspects of it as support of multitouch trackpad gestures, Recovery HD, hard drive encryption out of the box, nice backup solution built-in, awesome application ecosystem.

I use Linux daily at home and at work, but Windows 7 is probably my favorite:

Win7 is an extremely polished, and fast OS. everything just works out of the box and the UI (atleast on MS apps) is intuitive, except for maybe Windows Media.

I have a lot of gripes with OS X, especially the workflow. I find that it's slower to navigate around than Windows, especially if you don't have a trackpad -- OSX heavily relies on its touchpad to be efficient. The keybindings/macros aren't that great either, and are cumbersome and annoying to use. I also have many gripes with Finder. It looks pretty but it's not as efficient as other OS' window managers

Most of my time has been spent in Windows 7, although I've always dabbled in Ubuntu. Recently, inexplicable errors, winrot, etc. are pushing me to use Ubuntu more often.

In my experience, technical Windows forums hardly ever manage to fix tough problems, although, to be fair, I've never yet experienced any such problems on Ubuntu.

For me, OS X. Great design, speed, stability... It has all the apps I need, it's great for development and under the hood is still a Unix machine with a lot of CLI utils.

But I really like Windows 7 and Ubuntu too. It's just that I like OS X more.

I'm stuck with Windows as my favourite image manager, ACDSee, doesn't work all that well under Wine.

When I want to do online banking, I boot into Linux.