Ubuntu
Version 13.04
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 and the issue will be 'fixed' with the 100 Scopes service in Ubuntu 13.10...although whether it will be included in Unity on XMir I don't know.
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: 5/10 (I can't really decide on this one, it's slow (admittedly), but not quite as slow as Vista. It does the job, but even on my 2.5GHz Dual-Core PC, it still takes a while for the Dash to load (which is understandable because it is searching your whole PC all the time). In my 2GB RAM VirtualBox however, it is pretty much unusable whereas Lubuntu IS REALLY fast. Hopefully, with Unity 8 (Ubuntu 14.10) which will be ONLY based on C++/QML, this will be faster, much faster.)
Overall: 7/10 (This seems like a low score, but it is still 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.