It's hard to answer this question having now beat the game and kowing each dungeon inside out. I personally believe it was Inside the Deku Tree although I did get stuck with trying to break the spider web to descend when I was younger. Also, none of the puzzles were particularly perplexing-shooting eye doors with your slingshot, moving blocks, and reflecting Deku Scrubs' shots in a particular order. The boss battle was quite perplexing for me the first time around, however, it is a joke now especially seeing quick speed runs of it on the Internet. I do believe King Dodongo is a tab easier though.