At the expense of sounding like a parrot,
Zelda II
Quite frankly the skeleton goons would give me enough trouble since hitting them required exact timing. Never finished that game. For that matter I have never finished the original Zelda (second quest) either. Though it wasn't difficulty it was simply couldn't find some of the temples. And I will never resort to some walkthrough.
Currently I'm playing though the only Zelda that I haven't played which is Windwaker. And it's challenging but no where near as difficult or frustrating as Zelda II