Wow. That was the only word that came to mind when I set my eyes upon this image. To build a normal settlement is hard but to build one around the tree in Sanctuary, as well as make it look genuinely well-designed and liveable, is no easy feat.
Sadly this is the only image available of this incredible settlement and that is why it doesn't feature higher in this list, but it certainly channels the inner child in all of us! Thanks to Qz7624 on Reddit.