For me Mission Impossible for the N64 was a hard one, (I think I finished that one) also Goldeneye was tough to make it all the way through, especially "terminal" or whatever that one towards the end was called.
What about you guys? GameZone
easy, rama for the pc. it's like eight years old, but you had to learn base eight and base sixteen math, as well as some calculus to progress through multiple sections of the game.