If we're talking about NES games that kicked ass, I've gotta mention Ikari warriors 3 (or Ikari III).
That's a horizontal Shoot-them-up (most of the time beat-'em-up) game where you play a Rambo-like shirtless fighter, and have to fight through an army of enemy soldiers in order to save some girl (probably a President's daughter).
The level designs are really nice and varied, and so are the bosses. In the first level you would have to destroy a tank, and one of the final bosses is a heavily-armored train.
I don't know if to call this game easy or hard, because, because you only have one life in the game and die rather easily, but you have infinite continues, and continue from the very same point where you died.