What's the lightest Ubuntu desktop environment?

Definitely Lxde or Xfce.

First remove Unity with this command (make a backup before proceeding for safety!):

sudo apt-get autoremove --purge unity unity-common unity-services unity-lens-\* unity-scope-\* unity-webapps-\* gnome-control-center-unity hud libunity-core-6\* libunity-misc4 libunity-webapps\* appmenu-gtk appmenu-gtk3 appmenu-qt\* overlay-scrollbar\* activity-log-manager-control-center firefox-globalmenu thunderbird-globalmenu libufe-xidgetter0 xul-ext-unity xul-ext-webaccounts webaccounts-extension-common xul-ext-websites-integration gnome-control-center gnome-session

then:

sudo rm /usr/lib/thunderbird-addons/extensions/[email protected]

Now, to install Lxde run this command:

sudo apt-get install lubuntu-desktop

or for Xfce this one:

sudo apt-get install xubuntu-desktop

Lastly, reboot and enjoy!

Hope this helped you.

The real answer is: NO Desktop Environment.

Depending on what you want to do, running shell only might be a decent choice.

You can use something like fbterm to split your screen into multiple sections

And there are shell programs that allow you to surf the web, view PDFs and play music as well.

Disclaimer: That isn't my desktop screenshot, My shell only laptop is a work in progress, however I know someone who has successfully set this up, so I know what I'm talking about.

Disclaimer 2: I don't think the shell web browsers play well with Stack Exchange.

As other users have answered, LXDE is the lightest option. My input, however, would be that it definitely makes sense to do a clean install of lubuntu as a whole OS, replacing your ubuntu system.

The lubuntu OS comes packaged with many lightweight programs that maximize efficiency (though you can still install all of your heavier favorites because lubuntu uses the same software sources as regular ubuntu), has very little bloatware, and the entire system is built from the ground up specifically to be lightweight. Also, many of the applications it comes with integrate very well with LXDE specifically. I think that this is the best way to get the most out of low-spec machines.

You can download it here and use unetbootin to make a live CD or flash drive.