Hardest game is an easy choice, Mushihime-sama Futari, which is this: https://www.youtube.com/watch?v=7uON3zty3-Q From what I've seen, if I had to estimate, I'd say that probably less than 20 players have ever actually beaten this mode (Ultra) in this game... it's THAT hard. As in, nearly freaking impossible. The final boss is known to be possibly the single most difficult boss ever made.
If I had to choose an easiest game, it just might be the very first Kirby's Dream Land on the original Gameboy. I got that game, and had finished it in an hour or two of getting it, and.... that was it. Very short game, very very easy. I mean, granted, the Kirby series isnt exactly meant to be very difficult, but still.