I cannot believe there is anybody who doesn't believe that XCOM is objectively an incredible game. The only excuse I can think of was that it was too hard but I would bet my right hand that Dark Souls features on this list. If you haven't played this game I strongly suggest you do; it will likely be on sale during this Steam Sale and it is imperative that you try it. Play it on easy mode, if need be, but the challenge is what makes it marvellous.