Ubuntu
Version 14.04
Ubuntu, quite simply, is the best OS on the planet. It is logical and easy-to-use ('X' in top-left with the Dash Search and Launcher, so you can quickly close a program and then open a new one, notifications nearby in the top-right with all your social media coalesced into one button, your sound + music in another and system-based stuff in another - although there isn't much to see there because Ubuntu manages your PC without you having to do much other than install optional Updates which often don't require a restart like Windows Updates do - the Rubbish Bin is out of the way in the bottom-left and you have a traditional desktop).
The Dash Search is continuously getting better to the point that one day, they want it to be as intuitive as the computer in Star Trek. One controversial feature is the Amazon search results that come up automatically and irrelevantly in the Dash, but I actually like this feature because it is only a small strip of results at the bottom of the screen and makes Canonical more money to keep making Ubuntu great. There is also an option to turn these search results off.
Ubuntu is also great for power-users (albeit - a little slow and slightly less snappy than Windows or the real power-user desktop KDE) with it's HUD which, when you press Alt, let's you search for menu items instead of clicking on lots of submenus - saving time and great for the impatient. It is also quite clean in that the menu to each program is always in the thin top bar to the left of the notifications, hidden behind the name of the program - slightly less easy-to-use, but it uses space more effectively. If the program isn't maximized, the title bar shows the program name instead of the title provided by the program which is in the bar at the top of the non-maximized window instead: more useful.
Being the biggest Linux distro that will run on the most form factors (desktop, server, cloud, phone, tablet and TV) by April 2014, there are a fair few programs that run on it: enough to do everything you want it to do - for free. You can still run your old Windows programs OK in PlayOnLinux (Wine) and many commercial programs have Linux versions (Steam, Lightworks (Beta)) probably because many developers work on Linux desktops anyway...
Usability: 9/10 (Some minor issues like the hiding of the 'X' in the corner and the lack of clarity that the Dash is the Ubuntu search (people don't know that they need to click on the Ubuntu logo) but otherwise the most powerful UI around)
Size: 7/10 (Too big to fit on a CD but is pretty good considering it is less than half the size of Windows)
Performance: 10/10 (I gave this a 5/10 when 13.04 was released because it was running slowly on my desktop but I have a laptop from about the same time (still older than most peoples') and, actually, it is VERY fast. Boots in 40s (on this old thing, that's FAST) and is extremely responsive. No problems at all and, if there is, that's due to a lack of RAM or CPU power, the interface itself is not at fault. I imagine that even 'lightweight' interfaces wouldn't run much faster on this laptop)
Overall: 9/10 (It is the operating system that you should run. It is the most popular desktop Linux operating system and it will exist on every other form factor as well by 2014. This IS the future of computing)
Written by Adam.