Papers Please - Remember the whiskey
Papers Please is one of those games which on the face of it is easy to explain - you play the role of a border crossing guard, who checks the passports and documents of immigrants. The subtleties of the game however are harder to describe. The simple art style with washed out colours and the oppressive music all add to an atmosphere which is rather depressing, as you battle with the balance of doing your job right, taking bribes, feeding your family and deciding whether or not to undermine the regime which employs you.
After playing, you feel that you understand what it must have been like in the mid 80s to be part of an eastern bloc country (I am sure it doesn't actually equip you with this knowledge, but it feels like it does). This is why you need to remember to bring a glass of whiskey when playing - you will need it.