I chose the original Legend of Zelda because you took a lot of damage going from screen to screen if you were not careful and then you would have to enter a dungeon with little life. The enemies that threw the swords were annoying and took forever to kill if you didn't have the upgraded sword. A Link to the Past was my first Zelda game and I don't think it was really that difficult. I got through the game fairly easily.
Very interesting that you chose to exclude Zelda II: Adventure of Link as well as Majora's Mask. We'll be sure to see quite some diversity in terms of replies in this thread.
With those two choices aside, I thought the hardest Zelda game is A Link to the Past. Starting from as early as the third dungeon, The Tower of Hera, the difficulty level spikes significantly upward. After obtaining the Master Sword and travelling to the Dark World, however, things start to get very interesting. The overworld is filled with baddies around every nook and cranny. The dungeons are also very well designed with many secrets hidden in destructible walls as well as a confusing layout which truly requires the player to bust out the dungeon map. Bosses in 2D Zelda games may seem like a piece of cake with only a length and width plane to navigate but that could not be farther from the truth. Something the bosses in ALttP do a lot is move. In this respect, they are a lot harder than those in modern 3D titles who are stunned upon impact giving you time to slash away/ A Link to the Past truly is a masterpiece, however, to me it is something much more-A painful lesson in how hard games were during the golden era of 2D gaming.
The hardest Zelda game is Zelda II, however I don't think I have to explain. Besides Zelda II, I would have to go with ALTTP and TLoZ. Both games don't really tell you where exactly you have to go. In TLoZ, you were pretty much on your with a few hints here in there, but you had to explore and travel the land on your own which can lead you into a goose chase. In ALTTP, the game does tell you where to go, but it isn't that easy. Sometimes you need to take a special route to get into the dungeon. Basically, both games have a lot of exploration and you might not know where to go or what to do next. Enemies are also challenging. They can take away a lot of health and a lot of rooms are full of enemies. Dodging them can be hard since there are usually other things on the screen trying to kill you like the Beamos, Anti-Fairies, Bubbles, ect. You will probably die a lot too since you need fast reflexes in order to dodge the enemies. If I had to choose which game is harder, I would have to go with ALttP since it is a bigger game, however Zelda II is still harder.