Per this list of hardest NES games the most difficult I’ve beaten is the original Mega Man. Not sure I agree with the ordering because I think that Battletoads, Ghosts N. Goblins, and Teenage Mutant Ninja Turtles 1 are all more difficult. I’ve played and been unable to beat them. Although I did borrow Mega Man for a few weeks, and the others I played on rental or over at a friend’s house schedules, so maybe I just didn’t get enough practice.