I gotta say when I was playing OoT hardly any bosses were hard for me the first time. There were two bosses that really gave me trouble the first time. First is Ganondorf because I had beat his phantom without knowing I could reflect what he shot at me, so when Ganondorf shot his blasts I was clueless as to what to do with them. (The mirror shiel was attempted... alot.) And the absolute hardest boss, which is actually a mini boss, is, and feel free to make fun of me, the giant Octorock in Lord Jabu Jabu. I couldn't get link to move fast enough to do what I wanted. Between the spinning center and the Octorock turning around, I spent forever on him, though I don't think I lost all my hearts. I don't remember if I even used the boomerang or not. The second time around I had trouble with Giant Octorock, and Bongo Bongos.
For all those who are saying Morpha, I think you were tired after the Water Temple, because when I saw the picture of the monster I thought it would be the coolest thing ever but when I fought it I thought "Were the people who made this game serious?" I couldn't believe how easy he was. Seemed to me like a break in the game after that long hard temple.