Best operating system?

It depends on the task you are trying to accomplish. That should be the deciding criteria
except vista no one should use vista

Windows 7 for me. I worked on multiple ultrabooks that had 8 and it can go burn in a fire. Plus, they are already leaking Win 9.
My limited experiences with IOS have mostly been frustrating and disappointing.

Windows 7 / Windows XP

Windows 7 Ultimate

xp

Windows 7 if you're into gaming and like to keep things simple.

Linux if you want to do some real thinkering and like to reinstall your entire system when you run into a problem because if you're new at this, you're probably end up doing more damage trying to fix things.

I used a machine with Windows 95 when I was really young, then started using dad's notebook (Windows XP), and now I'm on a PC with Windows 7. I also tried Ubuntu, but my favourite OS is W7.

I'd say Windows 7. I've used Windows 3.1, 95, 2000, ME, XP, Vista, 7, 8 & some version of Mac OS X (used it in school).

Windows 7, haven't had the chance to use 8 yet but it doesn't look too bad to me.

Linux

Has anyone noticed how Microsoft seems to have created a pattern with production quality?
BUY! -> Stay Away! -> Buy! -> Stay Away!
98SE-----WinME-------Win7------Win8

Windows 8 is actually not bad. Especially with Stardock's Start8 program. Basically gives you the option to boot to desktop and use the same start menu options and gui as Windows 7. I even think Windows 8 runs faster than Windows 7. Boots faster and other nice features.

Windows '95

Personally windows 8 is the best because I use a wide variety of programs (specifically I play games) and its faster with improvements over Windows 7. I dont use metro but I think the other improvements are worth clicking the desktop box whenever I turn my computer on.

For others... Linux covers a very wide variety of OS and some of them are very good at what they do.

My personal favorite at the moment is the PSP's OS. So simple! It plays games! It functions nearly perfectly.

Windows 3.11 for workgroups. I run it on my laptop.

Edit: I'll try and get a screenshot soon.

DoS

Windows 7 for most gaming

Linux for everything else

Windows 8 Pro.

Hmmmmm, not sure.

Even though I'm primarily using Windows 7, I would use Linux almost exclusively if I could run all my games and applications without WINE and compatibility issues. That said, Windows 7 is actually a pretty good OS, and I can't say it makes me miss Linux like XP and Vista did. OS X is interesting. It's supposed to be "easy" to use but I found myself using the terminal in it more than I do in Linux distro with a GUI. Windows 8 is actually better than Windows 7 under the hood, but the interface is revolting (though, actually, not as bad as Unity or GNOME 3). Not worth the money to upgrade, as a result.

For mobile devices, I stick with Android. iOS and Windows Phone are too locked down for my liking and the rest just don't have enough support.

Windows 98

Gentoo Linux for ma' servers and a desktop or two.
But for gaming; Windows 7, 8 is too messed up, Vista sucks, and XP is getting very outdated now and no DX10+ support

Windows 7