I Wanna Be The Guy, absolutely no question whatsoever. Majora's Mask, Castlevania, AoL, Mega Man... Those games are a complete cakewalk compared to the RAEG that the freeware PC game IWBTG induces. Basic summary of the game: You are the Kid. You want to be The Guy. To do this, you must kill The Guy. But first, you must go through an enormous dungeon, killing various bosses from an assortment of games and avoiding completely unpredictable obstacles. No exaggeration: Everything in the game wants to kill you. And no matter what hits you, you have exactly 1 HP. Meaning you die after any hit whatsoever. And the game is merciless.
Example #1: After killing one of the bosses, you must jump out of the sky and land in the water below. After avoiding a few obstacles here, you approach what looks like a giant wall. When you near it, the moon falls out of the sky and attempts to land on you. If you successfully avoid it, it begins to chase after you.
Example #2: You fall into the middle of a giant Tetris board and must climb your way out to the top right. If you take too long... Well, I won't spoil the absolutely hilarious and ironic moment, but let's just say if you take too long, you will be annihilated.
Example #3: In the Metroid area, if you enter the next room from the wrong location, a Metroid pops out of nowhere and consumes you.
Yeah, no contest- IWBTG is EVIL.
Other difficult games I've completed: Castlevania I/ Dawn of Sorrow/ Order of Ecclesia/ Portrait of Ruin, Heroes of Mana, Final Fantasy III, Shadow the Hedgehog, Majora's Mask, Metroid Prime I and III, Cave Story good ending, Mega Man 2, Super Mario Bros. 2