Mine is definitely OS X. I can't explain it well; I just like the way it is made and arranged. It's not the best for programming, but I still enjoy programming on it. Next, I guess would be Windows, even though it is very buggy. It is very customizable. It is my favorite for programming. I really like VS, Xcode too but my favorite would have to be VS. I also like using WinAPI and WinSock2, BSD Sockets aren't much different though. Linux is good, but it just seems really basic to me.
Linux is good, but it just seems really basic to me.
Anyways, my personal favorite is GNU/Linux... then Haiku.... then FreeBSD... then MINIX... then after that, maybe Plan 9 if I ever get around to actually messing with it.
I prefer Linux due to the monolithic kernel and philosophy. It also has sane support. There are things I don't like about it (like certain decisions with audio...).
I'd prefer Haiku next only because of it's choice of C++ in its core and interface, which is actually sanely done as well.
Then FreeBSD because of it's differences from Linux. I've not played with it enough to actually warrant me forming an opinion on it though.
Then Minix because it's probably the most used microkernel, sadly ahead of GNU/Hurd.
Windows will not be on this list for various reasons. It doesn't have a standardized filesystem layout. It's actually against a EULA to change the start icon since it requires patching resource files. You are restricted to a single shell (if it's even a shell at all). It's closed-source (I have no idea what Microsoft is taking from me as far as monitoring and information) while Microsoft themselves isn't very well known for having the consumers best interest in mind.
I use Windows when I want to play games and Linux when I want to program. If I'm doing anything else then I just use whichever, usually Windows. If Steam OS is a success and games start being made for Linux instead of (or as well as) Windows, though, I'll probably just stick Windows on a VM for older games and use Linux full-time. I'm not overly hopeful though.
I used to stay on and love Windows 7, mostly because of gaming and because of the MSDN.
As chrisname quoted, SteamOS has been announced, for this reason I just made a clean partition to put a Linux distro onto it, just to get myself back into linux, step by step.
But, quoting computerquip, the sound system isn't the best around.
And, quoting myself on another topic, the input system is a bit laggy - at least for me it is.