Well, the hardest game I ever played would be Ninja Gaiden 2 on the hardest difficulty and the name escapes me. Was it Ninja or Shinobi?
There was this one boss. This one fucking boss. It was this skeleton ogre thing about halfway through the game, I believe. He had this attack that was TOTALLY UNAVOIDABLE where he'd pick up Ryu, CHOMP HIS FUCKING HEAD, then throw him across the arena AT A WALL, effectively taking out at least 3/4ths of my health.
This boss was literally just dumb luck to beat, because I had to keep reloading until finally, during one battle, he didn't use that unavoidable attack until he was nearly dead. It was total bullshit and as soon as I finished the game I said "DONE" and sold it. Not even the Archdemon in either of his forms was as hard as that skeleton asshole.