I've never really played any old school games, but I've heard they were very hard. The ninja gaiden games I've heard are extremely hard, but again I haven't played any. I also won't take into account games that I'm not really into(3rd person shooters and stealth games off the top of my head).
So the hardest game I've played that didn't have different difficulties(so that's like a medium difficulty, right?) was Jak 2. Super hard game, especially because I got it right when it came out when I was 11. I recently played it again and beat it, without TOO much trouble, although some parts are absolutely ridiculous.
Off the top of my head the game that was the hardest on the hardest difficulty was CoD: WAW. Solid spam grenades every level. How did they even throw 10 grenades at the same time at me???
Ninja Gaiden and Dead Space were really easy for me. But the hardest game I ever played has to be Prince Of Persia: The Warrior Within. The only reason I say this is because glitchedx at the end of the game so I couldn't actually win haha. I got it when it first came out. It bugged out a few times on me. So since it was impossible to beat at that point that makes it the hardest game ever. And I never finished it because I didn't want to play it over again.
Off the top of my head the game that was the hardest on the hardest difficulty was CoD: WAW. Solid spam grenades every level. How did they even throw 10 grenades at the same time at me???
Yeah the veteran for WaW is impossible even to some people on treyarch.
The legend of zelda windbreaker for the gamecube. I had no clue what to do like at all.
super mario bros 2 japan version because it wasnt released in europe and america because it was considered too hard lucky they released it on virtual console renamed super mario bros. the lost levels really hard!