I think the hardest video game ever is haunted castle. I only know of 2 people who have ever beaten it, let alone do a speed run online. There's only 3 continues and most enemies take away half your health. Once you use your 3 continues you have to start at the beginning of the game. If you want to try it I think you can play it on vizzed.com. I don't think you can play it anywhere else because the game normally costs over $1000 due to it's extreme rarity.
I think halo 4 on normal... Jk I don't think I even died on that. Oblivion for me because I didnt understand how that series works I tried to play it how I play fps games by running through it so anytime I saw an enemy they'd basically just look at me and I'd die. The guy saying assassins creed is hard wow I destroyed that game and only one spot where I was surrounded did it take me more than a couple tries. I only recently started gaming so I can't comment on all the old school stuff but I do know most of it was really hard
Conkers bad fur day is hard (but I made it ).
But theres nothing like some rage games (cat mario, unfair platformer... where your remaining lives count in minus (you got -134648 lives left) or you just got some huge number of deaths)