Xcom on Impossible difficulty, I tried to complete it so many times but gave up . Personally I didn't find The Evil Within that difficult, I think I only died like 4 times during the entire game, and most of those were due to me getting one hit killed by bosses or Rivik
I can second this. Even on Normal difficulty, X-COM gives no mercy, and more than once it just ate me alive despite all my experience...LIVE!