A game who's potential was cut short by the actions of a bad faith developer and who's decaying body is kept somewhat alive by the modding community. Insanity.

The Skyrim of survival sanbox games - Starbound has seemingly everything to make you want to finish it, but you never will.
You'll spend hours trying out different mods and potentially even succeed at molding the game into an overall pleasant experience, but you'll still never finish it.
You can even play it with friends, so that you'll definitely never finish it. The absurd decision to add a main questline to a sandbox game was just THAT bad for it, especially when you had sunk in most of your hours before it was added.

Fantastic game. Space terraria but infinitely customizable.