After some wait and development work, the Ubuntu 17.10 daily builds are shipping with default GNOME desktop environment. This way, Ubuntu has finally ditched the Unity desktop in the true sense, which had been the default choice since 2011.
At the moment, the LightDM is the Ubuntu display manager, but the final version will ship with GDM. LightDM has two sessions: GNOME and GNOME Wayland. These builds are early releases, so expect few bugs while trying them.