i beat knife party an hour ago! but the hardest for me is god of war because at the end the game cheats and you always fall for no reason. but i Beat It!
From my personal experiences, the hardest games are arcade games, as they are purposely set up that way to take as much of your money as they can. So, pretty much any arcade game.
I would say that Siberia 2 is the toughest game I've played in my life. I beat it in two months without any use of the game guide or cheats, it was really tough to play since you can interact with so many objects.
The First Ninja Gaiden For the original X-box. Probably not the hardest now but I was i little younger when i first played it and as i Remeber it was crazy hard.