I'm not the best gamer on the planet. Frankly I stink but I've beaten a few NES games that are tough. For casual gamers at least.
I've beaten all three Castlevanias, the first three Mega Mans and Metroid. All games I'd consider difficult but two games I've played a lot of and I just can't beat are Ghosts n' Goblins and Zelda 2. Those would be my two choices. I've heard Silver Surfer and Battletoads are very tough. What do you think the toughest games for NES are?