If The Legend of Zelda doesn't count then probably Zelda II: The Adventure of Link or Phantasy Star 1.
Wow. Zelda II is a doosy, too. I still can't beat it, most of the time. I can always make it to the final boss, but that damn Shadow Link gets me every time after being nearly drained by the Phoenix right before him.
I did beat it once, but that was a fluke.
Still love the game, though.