LOL. I agree with all the other posters that said Inside The Great Deku Tree. Super easy. It's definitely just a tutorial level. It's not time consuming, unless you count the opening cut scenes, and getting the Kokiri Sword, and Deku Shield. And even then, it's still easier, and shorter than the other dungeons.
Let's see.
You have to go see Princess Zelda, get the Keaton Mask for the guard(optional), buy the Hylian Shield, backtrack to the Kokiri Forest for Saria's Song(that includes going through the Lost Woods), and then play for Darunia, and then blow the freaking cavern open.
That's just to get in the dungeon.
See how much easier the first dungeon is? No comparison.
If you want to know which one is the hardest IMO(just to add to the conversation), I think it's Dodongo's Cavern. Only because I've played the game so many times, and I just enjoy the two other Dungeons so much more than Dodongo's Cavern, and everything I have to do to open it. -__-