The hardest game is, without doubt, Mario Party 1.
Allow me to explain myself.
Mario Party is not quite difficult in the traditional sense of the word. To beat story mode, your simple task is to defeat mini-games. There is, however, one extremely difficult mini-game that makes beating the rest of the game impossible. I'm not sure of its name, but it involves a river raft and savage shy guys. I've never been entirely sure why these vicious brutes want to spear the various characters. I happen to think that this game's programmers were mad. The wild shy guys do not make this level difficult. It is the control method. If, by the end of the mini-game, your palms are not blistered, there is clearly something wrong. After all the pain, you are afraid to turn the game on.
Mario Party 1. Hardest game ever.