In my 14 years of gaming, there is only one game that I can say is the hardest one I played. The first Ninja Gaiden on Xbox. I mean, the difficulty is just insane, I couldn't get past Chapter 10(I think it's Military Supply Base) even on Normal. But it's amazing regardless.
That game is the only one that I will ever consider the absolute masterpiece of gaming industry, mostly because there are so many restrictions. Hell, even the producer/director of Team Ninja(sorry, can't remember his name now) said in an interview, that he is drunk when making games.
Later, they went AWOL with Ninja Gaiden 2 and made it easy as hell(I don't know if he was on that project).
This is all my opinion, of course.