The hardest level or dungeon. That's a tough one. Honestly, I'm going to go with the Ice Cavern from Ocarina of Time. The Freezards were a pain. I'd wait for his ice breath to stop. But once it did, an Ice Keese would come and freeze me, taking a heart with it. Thinking I had enough time, I'd continue running to the Freezard once I thawed out. His breath would freeze me, and then an Ice Keese would hit. This happened over and over, and I hated it. Also, in my first playthrough, I had only one bottle at the time. This meant I'd constantly have to go back for blue fire. It was infuriating to go through this mini-dungeon; I consider it the hardest because it really is in the right(or in this case, wrong) circumstances.