For me, it must be the Great Palace from Zelda II. Everything is difficult about this dungeon.
The first thing to mention is the path leading to this dungeon. In order to reach the imposing Great Palace, the player must brave the Valley of Death. This area is, without a doubt, the most difficult overworld section in the game. Traversing ahead requires fighting difficult enemies and landing precise jumps. Any wrong move can send you into the lava, letting ine life go to waste. Once the player manages to brave the Valley of Death, they quickly find that this was only the beggining.
We now move on to the Great Palace itself. As the binding force that blocks your way disintigrates, the player takes their first steps into this labyrinth. This palace is unlike any other, as there are many dead ends with nothing to reward he player with. Finding one's way quickly proves to be a supremely difficult task, as the amount of pathways that can be taken is tremendous. The correct path through this maze of a dungeon requires dropping down invisible holes on the ground, of which there are many. Along with the confusing-enough task of navigating the dungeon, the player must encounter hordes of powerful enemies.
Not far into the dungeon, the player will sonn find that the Great Palace houses brutally powerful enemies not seen in any other palace. The first to note is the Fotail, a fire-spitting bird that likes to impede the player's progress in halways. As the player approaches, it will spit its fireballs more agressively, creating a hazard that is difficult to avoid. It also takes a good number of slashes to destroy. After the Fotail, there is still another much more powerful enemy, the Fokka. These bird knights are essentially Iron Knuckles with one devastating ability: a high jump. This makes fighting these creatures a tremendously hard task, as they can easily avoid your sword slashes. They can drain a large amount of the player's health and take a high number of hits themselves to defeat. The player's best bet may be to run, but even then, they can quickly persue the player to the end lf the screen. When the player manages to defeat the powerful enemies and navigate the dungeon, they come to find their greatest challenge.
As the end of the dungeon, as well as the game, nears, the difficult doesn't let up. This is proven by the two bosses of the Great Palace. The first of them is Thunderbird, which attacks by dropping fireballs rapidly. In order to even damage this boss, the player must cast the Thunder spell, draining much of their magic power. The player is now forced to conserve what remainging magic they have. Attacking Thunderbird is a hard task itself, as the flying beast is immune to both the upward and downward stab. As the player damages Thunderbird, it drops its fireballs more and more rapidly, not shoing any signs of weakness. Once the player manages to destroy this tough boss, they then come to their greatest challenge.
With Thunderbird gone, the player moves on to thr final screen of the game. The Triforce of courage is in sight, but there's on final test. The room dims, and Dark Link is summoned. The player now faces the final boss of Zelda II, who may very well be the most difficult boss in the entire series. He mirrors every move of the player and counters with supremely powerful attacks. Defeating him requires the full skill of the player with little to no mistake. Of course, there is an exploit to defeat him, but the player really doesn't deserve the Triforce of Courage after using this method. In any case, once Dark Link is vanquished, the Great Palace, and the game, is completed.
Looking back, the Great Palace really proves itself to be a great test to the player. It is easily what I think to be the most difficult dungeon in the series, bar none. I, myself, had a true sense of accomplishment after completing it.