How can I make LightDM the default display manager?

sudo dpkg-reconfigure lightdm

Will prompt you to make it default. More information here:

• https://wiki.ubuntu.com/LightDM

/etc/X11/default-display-manager should currently contain something else than lightdm.

Like I answered in another question, since LightDM starts on your machine, you should reference the binary in /etc/X11/default-display-manager :

sudo sh -c 'echo "/usr/sbin/lightdm" > /etc/X11/default-display-manager'