Hardest home console game for me has to be Battletoads. It's brutal from the 3rd level onwards. The Snakes, the rat race, all the vehicle levels are so difficult and varied. A brilliantly designed game but so difficult that very few ever get to experience the whole thing.
Hardest ever video game would be the US Donkey Kong arcade.
I think the numbers are something like; fewer than 5 people have ever reached the kill screen on an arcade cabinet and less than 10 if you include MAME.