Ok.. I think any classic Zelda gamer would agree with me that Dungeon 6 on the original LoZ was one of the hardest. Also, the final dungeon on the second quest. Try beating that without a guide, it's near impossible without having hours upon hours to strain yourself at it.
That is exactly what I would have said.
The level-6 in LOZ was the toughest dungeon in the game. (maybe the second after level-9) The level-7 that comes after it was A REAL JOKE compared to that dungeon.
Also, the last palace in AOL IS THE TOUGHEST DUNGEON of all time in Zelda. It's really hard to just even get to that palace. It is a really complicated dungeon (enough to become crazy, yes crazy. I even wrote the way to get to the boss on a paper not to forget it) and when you tough you finally arrived at the final boss (after several times trying to beat him, you're like YEAH! I finally kill him!), HOP, here comes the real boss and you're like HOW MY GOSH! (Can't recall how many times I had to do the whole dungeon again just to get beaten by this dumb thing.)
Now, I've beaten this game (AOL) three times (really good game by the way) and I still wrote the way to the boss on a piece of paper. (not to get by the same area for nothing) And the final boss is still a lot challenging.