The Iron freaking Knuckles in both Adventure of Link and Ocarina of Time. They charge at me and start swinging out their axes like madmen, and meanwhile I'm trying to figure out a way to penetrate their defenses. In Adventure of Link two Knuckles are stationed right before the boss fight in the first Temple; I loathe going up against them because my hearts are cut way down before making it to Horsehead. Ocarina of Time isn't as bad, but I dread them in this game since they're more reoccurring than the Stalchildren in Hyrule Field. I literally thought I'd fought twenty of them on my first playthrough, and while they might not fill me with actual fear, they are probably the most intimidating Zelda foes I've ever battled.