Legend of Zelda: Some of the later dungeons, I guess. They're not too bad, I suppose.
Zelda II: The Great Palace, easily the most difficult dungeon in the series.
A Link to the Past: I've only played this game, never owned it, never beaten it. So nothing, I guess.
Link's Awakening: Can't really say, I love this game.
Ocarina of Time: Running around Hyrule Field before learning the warping songs can be a bit of a bore.
Majora's Mask: Great Bay Temple, to an extent. Nothing that would keep me from replaying it, though.
Four Swords: Any part after the first five minutes I decide to play it, because it just gets boring after that.
The Wind Waker: The part where it's over.
Four Swords Adventures: I dread playing this game in singleplaer, because it feels like a watered-down version of the true multiplayer experience.
The Minish Cap: I found this game thoroughly enjoyable, so my answer would have to be nothing.
Twilight Princess: The City in the Sky. This dungeon is boring and takes a while to complete.
Phantom Hourglass: Some of the later dungeons as well as going back through the Temple of the Ocean King.
Spirit Tracks: The final battle. Not too hard, but also very irritating.
Skyward Sword: The whole thing. And that's why I haven't replayed it.