The only versions of Ubuntu I would recommend are those in active support and in practice that means 12.04 and 12.10 (since while 10.04 and 11.10 are still supported today support ends in April this year).
12.04 runs faster than 12.10 in my experience and 12.04 has the Unity 2D desktop which runs faster.
The reason I would not recommend older releases is that all support has stopped or is about to stop. You will find it difficult to install software since the repositories have been moved. Also there will be no updates either for bug-fixes or security problems.
Personally I would recommend Ubuntu 12.04 (LTS) it is supported till April 2017.
The Unity2D desktop is almost identical to the default one but it is faster. If you like the desktop as it was in say 10.04 you can get something similar to that with the following in a terminal. This command also works in 12.10
sudo apt-get install gnome-panel
Log out then back in selecting "Gnome Classic" or "Gnome Clasic (No Effects)" as your desktop from the login screen by clicking the circular icon next to the user name and the password entry box on the login screen. You can select the "Unity 2D" desktop this way too, though there is nothing to install.
Finally there are the Xubutu and Lubuntu variants, which are lighter and so run faster, but you have already said you don't like these as much.