Android
Version 4.0.5 Ice Cream Sandwich | Allwinner A13
Android (simply put) is the best production mobile OS in use today. It is mostly FOSS and developed and commercialised by Google, the world's most privacy-abusing but innovative and cheap yet very profitable company. Personally I don't like Google's inconsistent style (minimalist-yet-bloated browser, standard phone OS and cloud-based desktop OS) however they have made open-source very commonplace.
I am writing from a £35 tablet (search 'Allwinner A13 7"'in Amazon) which (although slow (which I am used to)) is probably the biggest bargain in the universe, seriously. It has decent specs for a tablet and does the same thing as the iPad (albeit slower) FOR A TENTH OF THE PRICE. Other Android tablets are also cheap in comparison to the Surface and the iThings. The main reason for this is that Android is FOSS so any company can use the OS and create lots of competition.
The OS works brilliantly on the low end (like my tablet) and in a speed test I conducted, loaded Temple Run 2 a whole THREE SECONDS faster than an iPhone 3GS with NO LAG. One of the reasons for this is that Google's alternative to the Java Virtual Machine (Dalvik VM) is DESIGNED for the low-end. Java is almost worth using just for DVM...
The OS itself is brilliant, a meld of Google's previous two operating systems, Gingerbread and Honeycomb (made for phone and tablet respectively): ICS let's you use a more phone-like interface when in portrait and a more tablet-like interface when in landscape. Interestingly, Gingerbread is still the main Android OS (maybe because Google don't force people to upgrade)...
Yeah, Android is brilliant, it is like a FOSS, Google-based iOS with widgets, a much bigger proportion of free apps and gesture typing (thanks to Swype) which makes Swyping on touch almost as fast as typing on a keyboard. I am hooked on Ubuntu coffee so I think I'll prefer Canonical's unified Unity approach but that will probably be out 2014 (a small wait) and Firefox OS will be better for the low-end (by banishing Java finally in favour of HTML5). But in the meantime, if you want the best budget computer, then get the Allwinner A13 or get another epic Android device (the £200 Nexus 7 or £150 Kindle Fire (Kindle Shell on top of Android 4.0) also recommended).
Oh, and an Android/Firefox OS watch will be coming out in the summer, beating Apple to the smartwatch :D
Usability - 10/10 (This version of Android doesn't have Swype by default...no other OS has it at all... = no problems whatsoever)
Size - 7?/10 (No problems I can see but it is rather featureful and uses Java extensively so it must be fairly big?)
Performance - 10/10 (Dalvik makes this one of the fastest OSes on the planet)
Overall - 9/10 (Depends on Java slightly too much and can be out-of-date on many machines but even then, it is an amazing OS. Biggest (?) phone OS, second biggest tablet OS, joint first smartwatch OS. One of the greatest FOSS victories.)
Written by Adam.