My question is simple: which edition of ubuntu is the most stable, reliable, fast and nice? And also, should I prefer Ubuntu, Kubuntu, Lubuntu or Mythbuntu?
Note: I want it to be as fast as possible.
Note : I will install ubuntu with wubi.
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I can not talk about MythBuntu, because I have no experience with it, but, as far as the other three go, they are essentially the same core operating system with a different graphical user interface.
Those three are very stable, and, performance-wise, the Lubuntu variant must be the most lightweight.
[EDIT2]: MythBuntu seems to be a home entertainment variant of Ubuntu.
Also keep in mind, that a strong pick, when it comes to performance too, is Xubuntu which is using XFCE as its graphical user interface. Speaking from experience, I find Xfce to be lighter than LXDE and more user friendly.
If your system should be fast, you could choose a lightweight desktop like LXDE, XFCE or another window-manager like fluxbox.
By experience, the latest will always be more stable. This does not count when we are talking about a very old system, but assuming the user is up to date with the hardware, the more recent the version is, the more updated it is, the more stable it is.
For each new version, there are many fixes, updates and compatibility issues attended. So it is normal to assume the latest is always better (Again, assuming the user updates his/her hardware).
Some improvements might be faster startups or cpu/memory usage, more hardware compatibility and file system enhancements, all of which add to the stable part.
Which is more recommended, depends on your needs and you hardware:
Ubuntu - Most tested and used. I personally recommend this one.
Kubuntu - More options, the interface looks similar in somes aspects to Windows (The type of menu and how/where options are.
Lubuntu/Xubuntu - For users with low hardware. Great speed and low cpu/memory usage.
Mythbuntu - If you like movies, tv series, music this is the one for you.
There are many others but again, this depend on how the user will use the system and the hardware they have.
Note about Wubi install
If you are going to install it by Wubi then you will not achieve the fastest performance. There is an excellent question/answer HERE and HERE about why NOT to use Wubi if you want to avoid several odd problems and performance issues. If you still do then your Note1 will have issues with your Note2 and we don't want 2 notes angry at each other do we? ^^.