Demon's Souls without a doubt, it wasn't only incredibly hard but it was also excruciatingly annoying especially when it sends you back all the way to the beginning of the stage when you die! - I do not want to play against flame lurker again! -
Also I found Resistance 3 and Wolfenstein the new order very hard but that is mainly because I'm not good at FPS games.
Demon's Souls was hard to plat. BUT. That ring. The Theif's ring I think it was called. It made 2 boss fights much easier. Including the last boss. I beat him 2 out of the required 4 times without it. Then, on the third fight, he was one hit killing me, and it became a chore to have to start over repeatedly, so I put it on and killed him with arrows with literally no resistance. I pussed out. Not proud, but I'll admit to it. Curious if you or anybody else used the ring to exploit the 2 bosses. I believe the other boss it worked on was Flame Lurker.
Chinese food for breakfast