Also,which will be the most appropriate for my old system?
Here is a small list:
This is the KDE flavor of the typical Ubuntu (which is GNOME driven) the primary difference is using KDE as the Desktop Manager
This is a lighter weight, highly efficient an optimized version of Ubuntu designed to run on older computers. It uses XFCE which is a proven faster Desktop Environment then both KDE and GNOME. This is a typically simpler slim interface.
The counterpart to Xubuntu - Mythbuntu is designed to be an entertainment powerhouse. Focused around being a Media Center it includes many drivers and setups for TV Tuners, TV Out cards, and has a Media Center application (MythTV) integrated into the Desktop Environment to facilitate the entertainment powerhouse it advertises.
Lubuntu takes the aims of Xubuntu and pushes the Desktop Manager to an even more bare bones lightweight variant: LXDE (Lightweight X11 Desktop Environment) which is supposed to be an even more efficient, power saving, fast, light weight Desktop Manager than XFCE.
I've used the first three before and I believe all have some backing from Canonical (or other corporate backing) while Lubuntu is still relatively new and community driven. Though if you have a very old/slow computer it certainly might be what you're looking for.
Figured I'd add the other flavors of Ubuntu I knew where out there
In addition to the above listed there is also:
This flavor is designed with Educational intent. Runs very close to the vanilla Ubuntu release though it comes with many applications that are best suited for an Educational environment. It also is configured and stylized with kids in mind.
This flavor is geared towards those who deal with multimedia (Video, Audio, Graphics, Design) on a daily basis. Comes bundled with many applications, codecs, and drivers required to facilitate those activities.
Ubuntu CE (Ubuntu Christian Edition) is Ubuntu designed for Christians who wish to maintain an "Christianly" lifestyle. This comes bundled with many religious softwares and security tools to help protect moral religious obligations.
Last I checked this project was idle - but it's intent was bring stronger hardened security to the stock Ubuntu installation
This flavor was designed similar to Kubuntu, Xubuntu, and Lubuntu in it was replacing the default GNOME Window Manager with an alternative - in this case Flux which is another alternative Window Manager for Linux
Only Kubuntu, Xubuntu, Edbuntu are recognized by Canonical. Mythbuntu is supported by a commercial entity though I'm not sure the name ( I believe it's the same company that backs MythTV )
The various different versions of Ubuntu differ only by branding and the default set of packages. On the inside all of the variations are similar and can be easily extended to include packages from each other just by going to the package manager and installing the packages you want.
So in a way there aren't that many differences between them.
On the other hand by default they focus on different things. Kubuntu focuses on KDE tools while Ubuntu focuses on Gnome tools and so on. So in the end it's mostly about your preferences.
Which one is most suitable for your old system depends a lot on how old the system is and what kind of hardware it has. The lightest ones of the bunch are lubuntu and xubuntu, not that others are too demanding either.
If I can, I would like to make a correction here. Ubuntu and Kubuntu receive official support from Canonical, the corporation behind Ubuntu. Xubuntu has not ever received support, although it is recognized as an official derivative. The developers of Xubuntu do so on their own time as volunteers. For use in an older computer, I would suggest either Xubuntu or Lubuntu, both of which are designed to use fewer resources than Ubuntu and Kubuntu.