Hardest game? Tetris probably.
There's a whole list of games that are straight up infuriatingly "cheap" that come to mind too (Ninja Gaiden 2 on the Xbox 360 for instance). Get hit once and you're in the middle of an enemy's 20 hit combo...
I beat the xbox Ninja Gaiden on normal and the 360 Ninja Gaiden 2 on normal. I'm not quite gutsy enough to try them on the harder settings, they were difficult enough on normal. In Ninja Gaiden 2 my game kept locking up on a video on like level 11 or 12 and I even exchanged the disk and it didn't work. I thought I was gonna have to start over and I almost cried. A game update fixed it luckily like 2 weeks later.
I remember having a friend tell me that Viewtiful Joe was crazy hard, never tried it myself though.
Ninja Gaiden/Black weren't that hard really, you just had to learn the combo system and learn to block/counter. NG2 though was awful with the enemy cheapness.
I beat Viewtiful Joe on the Normal difficulty, it wasn't bad. Never tried it above that though.