First of all, I need to warn you that Ubuntu comes pre-packaged with both Unity and Gnome, and many applications & features depend on them or their libraries. If you decide to follow any of the instructions below, you're doing it at your own risk. If you decide to remove those packages, it may be a good idea to run a simulation first. It will give you an idea of what will be removed. You can run simulation by adding the -s flag, eg:
sudo apt-get -s --purge remove package_name
To get rid of Unity (Ubuntu's default session), you have to remove unity packages:
sudo apt-get --purge remove unity*
As you may notice, there is a --purge flag. Adding it will also remove all associated configuration files.
To remove Gnome, your best option would probably be this:
sudo apt-get --purge remove gnome-*
Once again, I need to stress that many popular applications depend on Gnome's libraries and may refuse to work or be uninstalled along with Gnome.
Finally, in order to remove LXDE, your would probably run:
sudo apt-get --purge remove lxde*
As for the remaining applications, they are probably independent from Unity and you will have to remove them manually.